git-gui: Remove the commit_active global variable.

git-gui: Remove the commit_active global variable.

We were originally trying to use $commit_active to tell us if there was
a commit currently in progress, just so we didn't attempt to start a
second (parallel) one by mistake.  But really the index lock handles
this for us as it won't let us lock the index if it is already locked
for update.  So this can't happen.

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