git-gui: Setup branch.remote,merge for shorthand git-pull

git-gui: Setup branch.remote,merge for shorthand git-pull

When creating new branches if branch.autosetupmerge is not set, or
is set to true or always and we have been given a remote tracking
branch as the starting point for a new branch we want to create the
necessary configuration options in .git/config for the new branch
so that a no argument git-pull on the command line pulls from the
remote repository's branch.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>