git-gui: Unlock the index when cancelling merge dialog

git-gui: Unlock the index when cancelling merge dialog

Pressing the escape key while in the merge dialog cancels the merge
and correctly unlocks the index.  Unfortunately this is not true of
the Cancel button, using it closes the dialog but does not release
the index lock, rendering git-gui frozen until you restart it.  We
now properly release the index lock when the Cancel button is used.

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