when private journal file exists, still read from git-annex branch

when private journal file exists, still read from git-annex branch

Fix bug that caused stale git-annex branch information to read when
annex.private or remote.name.annex-private is set.

The private journal file should not prevent reading more current
information from the git-annex branch, but used to.

Note that, overBranchFileContents has to do additional work now, when
there's a private journal file, it reads from the branch redundantly
and more slowly.

Sponsored-by: Jack Hill on Patreon