mergetool-lib: combine vimdiff and gvimdiff run blocks

They are nearly identical outside of the foreground flag, which can safely
be passed to both vim and gvim. The merge tool itself is named in
$merge_tool_path.

Signed-off-by: Dan McGee <dpmcgee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-mergetool--lib.sh

@@ -169,25 +169,14 @@ run_merge_tool () {
 			"$merge_tool_path" "$LOCAL" "$REMOTE" | cat
 		fi
 		;;
-	vimdiff)
-		if merge_mode; then
-			touch "$BACKUP"
-			"$merge_tool_path" -d -c "wincmd l" \
-				"$LOCAL" "$MERGED" "$REMOTE"
-			check_unchanged
-		else
-			"$merge_tool_path" -d -c "wincmd l" \
-				"$LOCAL" "$REMOTE"
-		fi
-		;;
-	gvimdiff)
+	vimdiff|gvimdiff)
 		if merge_mode; then
 			touch "$BACKUP"
-			"$merge_tool_path" -d -c "wincmd l" -f \
+			"$merge_tool_path" -f -d -c "wincmd l" \
 				"$LOCAL" "$MERGED" "$REMOTE"
 			check_unchanged
 		else
-			"$merge_tool_path" -d -c "wincmd l" -f \
+			"$merge_tool_path" -f -d -c "wincmd l" \
 				"$LOCAL" "$REMOTE"
 		fi
 		;;