detect and recover from branch push/commit race

Dealing with a race without using locking is exceedingly difficult and tricky.
Fully tested, I hope.

There are three places left where the branch can be updated, that are not
covered by the race recovery code. Let's prove they're all immune to the
race:

1. tryFastForwardTo checks to see if a fast-forward can be done,
   and then does git-update-ref on the branch to fast-forward it.

   If a push comes in before the check, then either no fast-forward
   will be done (ok), or the push set the branch to a ref that can
   still be fast-forwarded (also ok)

   If a push comes in after the check, the git-update-ref will
   undo the ref change made by the push. It's as if the push did not come
   in, and the next git-push will see this, and try to re-do it.
   (acceptable)

2. When creating the branch for the very first time, an empty index
   is created, and a commit of it made to the branch. The commit's ref
   is recorded as the current state of the index. If a push came in
   during that, it will be noticed the next time a commit is made to the
   branch, since the branch will have changed. (ok)

3. Creating the branch from an existing remote branch involves making
   the branch, and then getting its ref, and recording that the index
   reflects that ref.

   If a push creates the branch first, git-branch will fail (ok).

   If the branch is created and a racing push is then able to change it
   (highly unlikely!) we're still ok, because it first records the ref into
   the index.lck, and then updating the index. The race can cause the
   index.lck to have the old branch ref, while the index has the newly pushed
   branch merged into it, but that only results in an unnecessary update of
   the index file later on.

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