git-gui: Add Reset to the Branch menu.

git-gui: Add Reset to the Branch menu.

cehteh on #git noticed that there was no way to perform a reset --hard
from within git-gui.  When I pointed out this was Merge->Abort Merge
cehteh said this is not very understandable, and that most users would
never guess to try that option unless they were actually in a merge.

So Branch->Reset is now also a way to cause a reset --hard from within
the UI.  Right now the confirmation dialog is the same as the one used
in Merge->Abort Merge.

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