git-gui: Do not offer to stage three-way diff hunks into the index

git-gui: Do not offer to stage three-way diff hunks into the index

git-apply does not accept a patch that was generated as a three-way
combined diff format such as we see during merge conflicts.  If we
get such a diff in our diff viewer and try to send it to git-apply
it just errors out and the user is left confused wondering why they
cannot stage that hunk.

Instead of feeding a known to be unacceptable hunk to git-apply we
now just disable the stage/unstage context menu option if the hunk
came from a three way diff.  The user may still be confused about
why they cannot work with a combined diff, but at least they are
only confused as to why git-gui is not offering them the action.

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