git-gui: Grab the index lock while running pull.

git-gui: Grab the index lock while running pull.

The user must not modify the index while a git pull operation is running,
doing so might cause problems for the merge driver and specific strategy
being used.  Normally on the command line people are just really good and
don't try to run index altering operations while they are also running a
pull.  But in a slick GUI like git-gui we can't trust the user quite as
much.

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