git-gui: Automatically refresh tracking branches when needed

git-gui: Automatically refresh tracking branches when needed

If the user is creating a new local branch and has selected to use
a tracking branch as the starting revision they probably want to
make sure they are using the absolute latest version available of
that branch.

We now offer a checkbox "Fetch Tracking Branch" (on by default)
that instructs git-gui to run git-fetch on just that one branch
before resolving the branch name into a commit SHA-1 and making
(or updating) the local branch.

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