builtin/refs: add 'exists' subcommand

As part of the ongoing effort to consolidate reference handling,
introduce a new `exists` subcommand. This command provides the same
functionality and exit-code behavior as `git show-ref --exists`, serving
as its modern replacement.

The logic for `show-ref --exists` is minimal. Rather than creating a
shared helper function which would be overkill for ~20 lines of code,
its implementation is intentionally duplicated here. This contrasts with
`git refs list`, where sharing the larger implementation of
`for-each-ref` was necessary.

Documentation for the new subcommand is also added to the `git-refs(1)`
man page.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Meet Soni <meetsoni3017@gmail.com>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-refs.adoc

@@ -18,6 +18,7 @@ git refs list [--count=<count>] [--shell|--perl|--python|--tcl]
 		   [--contains[=<object>]] [--no-contains[=<object>]]
 		   [(--exclude=<pattern>)...] [--start-after=<marker>]
 		   [ --stdin | <pattern>... ]
+git refs exists <ref>
 
 DESCRIPTION
 -----------
@@ -38,6 +39,12 @@ list::
 	formatting, and sorting. This subcommand is an alias for
 	linkgit:git-for-each-ref[1] and offers identical functionality.
 
+exists::
+	Check whether the given reference exists. Returns an exit code of 0 if
+	it does, 2 if it is missing, and 1 in case looking up the reference
+	failed with an error other than the reference being missing. This does
+	not verify whether the reference resolves to an actual object.
+
 OPTIONS
 -------