"checkout A...B" switches to the merge base between A and B

"checkout A...B" switches to the merge base between A and B

When flipping commits around on topic branches, I often end up doing
this sequence:

 * Run "log --oneline next..jc/frotz" to find out the first commit
   on 'jc/frotz' branch not yet merged to 'next';

 * Run "checkout $that_commit^" to detach HEAD to the parent of it;

 * Rebuild the series on top of that commit; and

 * "show-branch jc/frotz HEAD" and "diff jc/frotz HEAD" to verify.

Introduce a new syntax to "git checkout" to name the commit to switch to,
to make the first two steps easier.  When the branch to switch to is
specified as A...B (you can omit either A or B but not both, and HEAD
is used instead of the omitted side), the merge base between these two
commits are computed, and if there is one unique one, we detach the HEAD
at that commit.

With this, I can say "checkout next...jc/frotz".

Signed-off-by: Junio C Hamano <gitster@pobox.com>