help.c: add a compatibility comment to cmd_version()

External projects have been known to parse the output of
"git version".  Help prevent future authors from changing
its format by adding a comment to its implementation.

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

help.c

@@ -397,6 +397,10 @@ const char *help_unknown_cmd(const char *cmd)
 
 int cmd_version(int argc, const char **argv, const char *prefix)
 {
+	/*
+	 * The format of this string should be kept stable for compatibility
+	 * with external projects that rely on the output of "git version".
+	 */
 	printf("git version %s\n", git_version_string);
 	return 0;
 }