git-gui: Support resolving conflicts via the diff context menu.

git-gui: Support resolving conflicts via the diff context menu.

If the file has merge conflicts, show a special version of the
diff context menu, which includes conflict resolution commands
instead of Stage Hunk/Line. This patch only supports resolving
by discarding all sides except one.

Discarding is the only way to resolve conflicts involving symlinks
and/or deletion, excluding manual editing.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>