HedgeDoc crashed with
`uncaughtException: ENOENT: no such file or directory`
on startup, when `.git/ref/heads` did not contain
a file for the current branch. This seems to happen
regularly with current Git versions.
This fixes the crash by first trying to use the `git` executable for
getting the current commit SHA (before running our own parsing code)
and introducing a separate check to prevent accessing a nonexistent
file in `.git/ref/heads`.
Signed-off-by: David Mehren <git@herrmehren.de>
b719ce79db