When we update the current branch, we must also append to HEAD's reflog
to keep them in sync.
This is a bit of a hack, but as git.git says, it covers 100% of
default cases.
Any well-behaved program should write a descriptive message to the
reflog whenever it updates a reference. Let's make this more prominent
by removing the version without the reflog parameters.
The frontend used to look at the file directly, but that's obviously not
the right thing to do. Expose it on the backend and use that function
instead.
git-core only writes to the reflogs of HEAD, refs/heads/ and,
refs/notes/ or if there is already a reflog in place. Adjust our code to
follow these semantics.
Whenever a reference is created or updated, we need to write to the
reflog regardless of whether the user gave us a message, so we shouldn't
leave that to the ref frontend, but integrate it into the backend.
This also eliminates the race between ref update and writing to the
reflog, as we protect the reflog with the ref lock.
As an additional benefit, this reflog append on the backend happens by
appending to the file instead of parsing and rewriting it.
