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: NShawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -738,6 +738,7 @@ set empty_tree {}
 set current_branch {}
 set is_detached 0
 set current_diff_path {}
+set is_3way_diff 0
 set selected_commit_type new
 
 ######################################################################
@@ -2444,6 +2445,9 @@ proc popup_diff_menu {ctxm x y X Y} {
 		}
 		set l "Stage Hunk For Commit"
 	}
+	if {$::is_3way_diff} {
+		set s disabled
+	}
 	$ctxm entryconf $::ui_diff_applyhunk -state $s -label $l
 	tk_popup $ctxm $X $Y
 }