git-gui: Refactor single_commit to a proc.
git-gui: Refactor single_commit to a proc.

This is a minor code cleanup to make working with what used to be the
$single_commit flag easier.  Its also to better handle various UI
configurations, depending on command line parameters given by the
user, or perhaps user preferences.

Signed-off-by: Shawn O. Pearce <f01032651c12a22b2103346ba96b3b6b231f9d3c@spearce.org>
destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git