mergetool--lib: Allow custom commands to override built-ins

Allow users to override the default commands provided by the
mergetools/* scriptlets.

Users occasionally run into problems where they expect to be
able to override the built-in tool names.  The documentation
does not explicitly mention that built-ins cannot be overridden,
so it's easy to assume that it should work.

Lift this restriction so that built-in tools are handled the
same way as user-configured tools.  Add tests to guarantee this
behavior.

A nice benefit of this change is that it protects users from
having future versions of git trump their custom configuration
with a new built-in tool.

C.f.:

http://stackoverflow.com/questions/7435002/mergetool-from-gitconfig-being-ignored
http://thread.gmane.org/gmane.comp.version-control.msysgit/13188
http://thread.gmane.org/gmane.comp.version-control.git/148267

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-mergetool--lib.sh

@@ -104,11 +104,47 @@ run_merge_tool () {
 
 	if merge_mode
 	then
-		merge_cmd "$1"
+		run_merge_cmd "$1"
+	else
+		run_diff_cmd "$1"
+	fi
+	return $status
+}
+
+# Run a either a configured or built-in diff tool
+run_diff_cmd () {
+	merge_tool_cmd="$(get_merge_tool_cmd "$1")"
+	if test -n "$merge_tool_cmd"
+	then
+		( eval $merge_tool_cmd )
+		status=$?
+		return $status
 	else
 		diff_cmd "$1"
 	fi
-	return $status
+}
+
+# Run a either a configured or built-in merge tool
+run_merge_cmd () {
+	merge_tool_cmd="$(get_merge_tool_cmd "$1")"
+	if test -n "$merge_tool_cmd"
+	then
+		trust_exit_code="$(git config --bool \
+			mergetool."$1".trustExitCode || echo false)"
+		if test "$trust_exit_code" = "false"
+		then
+			touch "$BACKUP"
+			( eval $merge_tool_cmd )
+			status=$?
+			check_unchanged
+		else
+			( eval $merge_tool_cmd )
+			status=$?
+		fi
+		return $status
+	else
+		merge_cmd "$1"
+	fi
 }
 
 list_merge_tool_candidates () {