ref: add symlink ref content check for files backend

Besides the textual symref, we also allow symbolic links as the symref.
So, we should also provide the consistency check as what we have done
for textual symref. And also we consider deprecating writing the
symbolic links. We first need to access whether symbolic links still
be used. So, add a new fsck message "symlinkRef(INFO)" to tell the
user be aware of this information.

We have already introduced "files_fsck_symref_target". We should reuse
this function to handle the symrefs which use legacy symbolic links. We
should not check the trailing garbage for symbolic refs. Add a new
parameter "symbolic_link" to disable some checks which should only be
executed for textual symrefs.

And we need to also generate the "referent" parameter for reusing
"files_fsck_symref_target" by the following steps:

1. Use "strbuf_add_real_path" to resolve the symlink and get the
   absolute path "ref_content" which the symlink ref points to.
2. Generate the absolute path "abs_gitdir" of "gitdir" and combine
   "ref_content" and "abs_gitdir" to extract the relative path
   "relative_referent_path".
3. If "ref_content" is outside of "gitdir", we just set "referent" with
   "ref_content". Instead, we set "referent" with
   "relative_referent_path".

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fsck.h

@@ -86,6 +86,7 @@ enum fsck_msg_type {
 	FUNC(MAILMAP_SYMLINK, INFO) \
 	FUNC(BAD_TAG_NAME, INFO) \
 	FUNC(MISSING_TAGGER_ENTRY, INFO) \
+	FUNC(SYMLINK_REF, INFO) \
 	FUNC(REF_MISSING_NEWLINE, INFO) \
 	FUNC(SYMREF_TARGET_IS_NOT_A_REF, INFO) \
 	FUNC(TRAILING_REF_CONTENT, INFO) \