git-gui: added config gui.gcwarning to disable the gc hint message

git-gui: added config gui.gcwarning to disable the gc hint message

On startup in multicommit mode git-gui checks to see if the repository
has a lot of objects. If so it shows a dialog suggesting gc be run.
This adds 'gui.gcwarning' as a control config variable to allow this
to be disabled. The default is true (the warning is shown). Setting this
false will prevent the check being done.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>