add-patch: classify '@' as a synonym for 'HEAD'

Currently, (restore, checkout, reset) commands correctly take '@' as a
synonym for 'HEAD'. However, in patch mode different prompts/messages
are given on command line due to patch mode machinery not considering
'@' to be a synonym for 'HEAD' due to literal string comparison with
the word 'HEAD', and therefore assigning patch_mode_($command)_nothead
and triggering reverse mode (-R in diff-index). The NEEDSWORK comment
suggested comparing commit objects to get around this. However, doing
so would also take a non-checked out branch pointing to the same commit
as HEAD, as HEAD. This would cause confusion to the user.

Therefore, after parsing '@', replace it with 'HEAD' as reasonably
early as possible. This also solves another problem of disparity
between 'git checkout HEAD' and 'git checkout @' (latter detaches at
the HEAD commit and the former does not).

Trade-offs:
- Some of the errors would show the revision argument as 'HEAD' when
  given '@'. This should be fine, as most users who probably use '@'
  would be aware that it is a shortcut for 'HEAD' and most probably
  used to use 'HEAD'. There is also relevant documentation in
  'gitrevisions' manpage about '@' being the shortcut for 'HEAD'. Also,
  the simplicity of the solution far outweighs this cost.

- Consider '@' as a shortcut for 'HEAD' even if 'refs/heads/@' exists
  at a different commit. Naming a branch '@' is an obvious foot-gun and
  many existing commands already take '@' for 'HEAD' even if
  'refs/heads/@' exists at a different commit or does not exist at all
  (e.g. 'git log @', 'git push origin @' etc.). Therefore this is an
  existing assumption and should not be a problem.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Phillip Wood <phillip.wood123@gmail.com>
Signed-off-by: Ghanshyam Thakkar <shyamthakkar001@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

add-patch.c

@@ -1729,14 +1729,6 @@ int run_add_p(struct repository *r, enum add_p_mode mode,
 	if (mode == ADD_P_STASH)
 		s.mode = &patch_mode_stash;
 	else if (mode == ADD_P_RESET) {
-		/*
-		 * NEEDSWORK: Instead of comparing to the literal "HEAD",
-		 * compare the commit objects instead so that other ways of
-		 * saying the same thing (such as "@") are also handled
-		 * appropriately.
-		 *
-		 * This applies to the cases below too.
-		 */
 		if (!revision || !strcmp(revision, "HEAD"))
 			s.mode = &patch_mode_reset_head;
 		else

builtin/checkout.c

@@ -1224,7 +1224,9 @@ static void setup_new_branch_info_and_source_tree(
 	struct tree **source_tree = &opts->source_tree;
 	struct object_id branch_rev;
 
-	new_branch_info->name = xstrdup(arg);
+	/* treat '@' as a shortcut for 'HEAD' */
+	new_branch_info->name = !strcmp(arg, "@") ? xstrdup("HEAD") :
+						    xstrdup(arg);
 	setup_branch_path(new_branch_info);
 
 	if (!check_refname_format(new_branch_info->path, 0) &&

builtin/reset.c

@@ -281,7 +281,9 @@ static void parse_args(struct pathspec *pathspec,
 			verify_filename(prefix, argv[0], 1);
 		}
 	}
-	*rev_ret = rev;
+
+	/* treat '@' as a shortcut for 'HEAD' */
+	*rev_ret = !strcmp("@", rev) ? "HEAD" : rev;
 
 	parse_pathspec(pathspec, 0,
 		       PATHSPEC_PREFER_FULL |

t/t2016-checkout-patch.sh

@@ -38,26 +38,32 @@ test_expect_success 'git checkout -p with staged changes' '
 	verify_state dir/foo index index
 '
 
-test_expect_success 'git checkout -p HEAD with NO staged changes: abort' '
+for opt in "HEAD" "@"
+do
+	test_expect_success "git checkout -p $opt with NO staged changes: abort" '
 		set_and_save_state dir/foo work head &&
-	test_write_lines n y n | git checkout -p HEAD &&
+		test_write_lines n y n | git checkout -p $opt >output &&
 		verify_saved_state bar &&
-	verify_saved_state dir/foo
-'
+		verify_saved_state dir/foo &&
+		test_grep "Discard" output
+	'
 
-test_expect_success 'git checkout -p HEAD with NO staged changes: apply' '
-	test_write_lines n y y | git checkout -p HEAD &&
+	test_expect_success "git checkout -p $opt with NO staged changes: apply" '
+		test_write_lines n y y | git checkout -p $opt >output &&
 		verify_saved_state bar &&
-	verify_state dir/foo head head
-'
+		verify_state dir/foo head head &&
+		test_grep "Discard" output
+	'
 
-test_expect_success 'git checkout -p HEAD with change already staged' '
+	test_expect_success "git checkout -p $opt with change already staged" '
 		set_state dir/foo index index &&
 		# the third n is to get out in case it mistakenly does not apply
-	test_write_lines n y n | git checkout -p HEAD &&
+		test_write_lines n y n | git checkout -p $opt >output &&
 		verify_saved_state bar &&
-	verify_state dir/foo head head
-'
+		verify_state dir/foo head head &&
+		test_grep "Discard" output
+	'
+done
 
 test_expect_success 'git checkout -p HEAD^...' '
 	# the third n is to get out in case it mistakenly does not apply

t/t2020-checkout-detach.sh

@@ -45,6 +45,18 @@ test_expect_success 'checkout branch does not detach' '
 	check_not_detached
 '
 
+for opt in "HEAD" "@"
+do
+	test_expect_success "checkout $opt no-op/don't detach" '
+		reset &&
+		cat .git/HEAD >expect &&
+		git checkout $opt &&
+		cat .git/HEAD >actual &&
+		check_not_detached &&
+		test_cmp expect actual
+	'
+done
+
 test_expect_success 'checkout tag detaches' '
 	reset &&
 	git checkout tag &&

t/t2071-restore-patch.sh

@@ -44,13 +44,17 @@ test_expect_success PERL 'git restore -p with staged changes' '
 	verify_state dir/foo index index
 '
 
-test_expect_success PERL 'git restore -p --source=HEAD' '
+for opt in "HEAD" "@"
+do
+	test_expect_success PERL "git restore -p --source=$opt" '
 		set_state dir/foo work index &&
 		# the third n is to get out in case it mistakenly does not apply
-	test_write_lines n y n | git restore -p --source=HEAD &&
+		test_write_lines n y n | git restore -p --source=$opt >output &&
 		verify_saved_state bar &&
-	verify_state dir/foo head index
-'
+		verify_state dir/foo head index &&
+		test_grep "Discard" output
+	'
+done
 
 test_expect_success PERL 'git restore -p --source=HEAD^' '
 	set_state dir/foo work index &&

t/t7105-reset-patch.sh

@@ -26,12 +26,16 @@ test_expect_success PERL 'saying "n" does nothing' '
 	verify_saved_state bar
 '
 
-test_expect_success PERL 'git reset -p' '
-	test_write_lines n y | git reset -p >output &&
+for opt in "HEAD" "@" ""
+do
+	test_expect_success PERL "git reset -p $opt" '
+		set_and_save_state dir/foo work work &&
+		test_write_lines n y | git reset -p $opt >output &&
 		verify_state dir/foo work head &&
 		verify_saved_state bar &&
 		test_grep "Unstage" output
-'
+	'
+done
 
 test_expect_success PERL 'git reset -p HEAD^' '
 	test_write_lines n y | git reset -p HEAD^ >output &&