Before c9214c394b248e1f26e45dbe1be2bd82363af3a6 and
9d396d2e426b9a765de0b8999aa214f1259633e6 if .git is symlink the version
would gracefully default to no git revision. With those changes an
exception is thrown instead.
This introduces a new function `pathIsGitRepo` that checks if
`commitIdFromGitRepo` fails without error so we don't have to
reimplement this logic again and can fail gracefully.
46773a15b3