ref: add basic symref content check for files backend

We have code that checks regular ref contents, but we do not yet check the contents of symbolic refs. By using "parse_loose_ref_content" for symbolic refs, we will get the information of the "referent". We do not need to check the "referent" by opening the file. This is because if "referent" exists in the file system, we will eventually check its correctness by inspecting every file in the "refs" directory. If the "referent" does not exist in the filesystem, this is OK as it is seen as the dangling symref. So we just need to check the "referent" string content. A regular ref could be accepted as a textual symref if it begins with "ref:", followed by zero or more whitespaces, followed by the full refname, followed only by whitespace characters. However, we always write a single SP after "ref:" and a single LF after the refname. It may seem that we should report a fsck error message when the "referent" does not apply above rules and we should not be so aggressive because third-party reimplementations of Git may have taken advantage of the looser syntax. Put it more specific, we accept the following contents: 1. "ref: refs/heads/master " 2. "ref: refs/heads/master \n \n" 3. "ref: refs/heads/master\n\n" When introducing the regular ref content checks, we created two fsck infos "refMissingNewline" and "trailingRefContent" which exactly represents above situations. So we will reuse these two fsck messages to write checks to info the user about these situations. But we do not allow any other trailing garbage. The followings are bad symref contents which will be reported as fsck error by "git-fsck(1)". 1. "ref: refs/heads/master garbage\n" 2. "ref: refs/heads/master \n\n\n garbage " And we introduce a new "badReferentName(ERROR)" fsck message to report above errors by using "is_root_ref" and "check_refname_format" to check the "referent". Since both "is_root_ref" and "check_refname_format" don't work with whitespaces, we use the trimmed version of "referent" with these functions. In order to add checks, we will do the following things: 1. Record the untrimmed length "orig_len" and untrimmed last byte "orig_last_byte". 2. Use "strbuf_rtrim" to trim the whitespaces or newlines to make sure "is_root_ref" and "check_refname_format" won't be failed by them. 3. Use "orig_len" and "orig_last_byte" to check whether the "referent" misses '\n' at the end or it has trailing whitespaces or newlines. 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>