git-gui: Format tracking branch merges as though they were pulls

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 <spearce@spearce.org>