git-gui: Avoid unnecessary global statements when possible

git-gui: Avoid unnecessary global statements when possible

Running global takes slightly longer than just accessing the variable
via its package name, especially if the variable is just only once in
the procedure, or isn't even used at all in the procedure.  So this is
a minor cleanup for some of our commonly invoked procedures.

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