These magic files will be used to find out which commit / tree is mounted exactly.
Before, files with have a timestamp of 0 (Jan 1st, 1970), which can confuse programs (tar outputs warnings, for example). Now, all files on the entire filesystem use the commit timestamp (or the git-fs startup time, if no commit is available).
Unlike the previous git_reference_name_to_oid, this supports branch names, tags, trees, shas, etc. instead of just symbolic refs. Note that this function requires a libgit2 newer that 0.17 (i.e., this requires libgit2 from git).