git-gui: Correctly ignore '* Unmerged path' during diff.

If a path is really unmerged, such as because it has been deleted and
also modifed, we cannot obtain a diff for it.  Instead Git is sending
back '* Unmerged path <blah>' for file <blah>.  We should display this
line as-is as our tag selecting switches don't recognize it.

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

git-gui.sh

@@ -708,7 +708,8 @@ proc read_diff {fd} {
 		#
 		if {[string match {@@@ *} $line]} {set is_3way_diff 1}
 
-		if {[string match {index *} $line]} {
+		if {[string match {index *} $line]
+			|| [regexp {^\* Unmerged path } $line]} {
 			set tags {}
 		} elseif {$is_3way_diff} {
 			set op [string range $line 0 1]