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 <firstname.lastname@example.org>