Implement fast-forward when only tree (but not

From: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>

When the "bottom" commit and the HEAD don't match, but they refer to the
same tree (for instance after a refresh where only the description
changed), we can still fast-forward, by keeping the same top tree and
calling git-commit-tree (which only requires the tree object) with the new
parent. I've altered git.commit to allow this.

Btw, I've also avoided the use of .commitmsg and switched to piping the
description.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>