Open external merge tool with original file extensions for all three files

Before this change, conflicted files were open in external merge tool with
temporary filenames like REMOTE.$$ and LOCAL.$$.  This way meld was unable
to recognize these files and syntax highlighting feature was unusable.

Help such merge tools by giving temporar files the same extension as the
original.

Signed-off-by: Pini Reznik <pinir@expand.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-mergetool.sh

@@ -152,10 +152,11 @@ merge_file () {
 	exit 1
     fi
 
-    BACKUP="$path.BACKUP.$$"
-    LOCAL="$path.LOCAL.$$"
-    REMOTE="$path.REMOTE.$$"
-    BASE="$path.BASE.$$"
+    ext="$$$(expr "$path" : '.*\(\.[^/]*\)$')"
+    BACKUP="$path.BACKUP.$ext"
+    LOCAL="$path.LOCAL.$ext"
+    REMOTE="$path.REMOTE.$ext"
+    BASE="$path.BASE.$ext"
 
     mv -- "$path" "$BACKUP"
     cp -- "$BACKUP" "$path"