git-gui: Format tracking branch merges as though they were pulls
If we are merging a tracking branch we know exactly what remote URL that branch is fetched from, and what its name is on that remote repository. In this case we can setup a merge message that looks just like a standard `git-pull $remote $branch` operation by filling out FETCH_HEAD before we start git-merge, and then run git-merge just like git-pull does.
I think the result of this behavior is that merges look a lot nicer when the came off of local tracking branches, because they no longer say "commit 'origin/...'" to describe the commit being merged but instead now mention the specific repository we fetched those commits from.
Signed-off-by: Shawn O. Pearce <email@example.com>