completion: use __gitcomp_builtin in _git_difftool

Since we can't automatically extract diff options for completion yet,
difftool will take all options from $__git_diff_common_options. This
brings _a lot_ more completable options to difftool.

--ignore-submodules is added to $__git_diff_common_options to avoid
regression in difftool. But it's a good thing anyway even for other
diff commands.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pclouds

contrib/completion/git-completion.bash

@@ -1394,7 +1394,7 @@ __git_diff_common_options="--stat --numstat --shortstat --summary
 			--dirstat --dirstat= --dirstat-by-file
 			--dirstat-by-file= --cumulative
 			--diff-algorithm=
-			--submodule --submodule=
+			--submodule --submodule= --ignore-submodules
 "
 
 _git_diff ()
@@ -1435,11 +1435,11 @@ _git_difftool ()
 		return
 		;;
 	--*)
-		__gitcomp "--cached --staged --pickaxe-all --pickaxe-regex
-			--base --ours --theirs
-			--no-renames --diff-filter= --find-copies-harder
-			--relative --ignore-submodules
-			--tool="
+		__gitcomp_builtin difftool "$__git_diff_common_options
+					--base --cached --ours --theirs
+					--pickaxe-all --pickaxe-regex
+					--relative --staged
+					"
 		return
 		;;
 	esac