A while ago, qwx noticed that we clobbered the exec
bit when merging files. This is not what we want, so
we changed the operator precedence to avoid merging
dirty files implicitly.
But we do want to merge, because it's convenient for
maintaining permissions. So, instead, we should do a
3 way merge of the exec bit.
This patch does that, as well as reverting the rollback
of that change.
While we're here, we adjust the timestamps correctly
in git/branch.
This requires changes to git/fs, because without an open
handler, lib9p allows opening any file with any mode,
which confuses 'test -x'.
03e5d9e9e2