Make `magit-guess-branch' have git grep the reflog for at most one branch name.

This uses "git log --walk-reflogs --grep-reflog PATTERN, which is
supported in git >= 1.8.0.

Conflicts:
	magit.el
Merge branch 'feature/quick-reflog'
Merge branch 'master' into next
magit-refresh-log-buffer: fix section heading

Don't claim commits are in HEAD when they are not.
Merge branch 'master' into next