git-gui: Restore ability to Stage Working Copy for conflicts.

git-gui: Restore ability to Stage Working Copy for conflicts.

Tools like rerere leave files marked as conflicts in the index,
while actually resolving them in the working copy. Also, some
people like to use an external editor to resolve conflicts.

This patch restores functionality previously removed in
commit 617ceee653 by adding a new context menu item.
It still ensures that the user does not stage conflicting files
accidentally by clicking on the icon instead of the name.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Tested-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Shawn O. Pearce <sop@google.com>