git-fetch: Avoid reading packed refs over and over again

When checking which tags to fetch, the old code used to call
git-show-ref --verify for each remote tag. Since reading even
packed refs is not a cheap operation when there are a lot of
local refs, the code became quite slow.

This fixes it by teaching git-show-ref to filter out existing
refs using a new mode of operation of git-show-ref.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-fetch.sh

@@ -242,7 +242,7 @@ esac
 reflist=$(get_remote_refs_for_fetch "$@")
 if test "$tags"
 then
-	taglist=`IFS="	" &&
+	taglist=`IFS='	' &&
 		  echo "$ls_remote_result" |
 	          while read sha1 name
 		  do
@@ -438,17 +438,11 @@ case "$no_tags$tags" in
 	*:refs/*)
 		# effective only when we are following remote branch
 		# using local tracking branch.
-		taglist=$(IFS=" " &&
+		taglist=$(IFS='	' &&
 		echo "$ls_remote_result" |
-		sed -n	-e 's|^\('"$_x40"'\)	\(refs/tags/.*\)^{}$|\1 \2|p' \
-			-e 's|^\('"$_x40"'\)	\(refs/tags/.*\)$|\1 \2|p' |
+		git-show-ref --exclude-existing=refs/tags/ |
 		while read sha1 name
 		do
-			git-show-ref --verify --quiet -- "$name" && continue
-			git-check-ref-format "$name" || {
-				echo >&2 "warning: tag ${name} ignored"
-				continue
-			}
 			git-cat-file -t "$sha1" >/dev/null 2>&1 || continue
 			echo >&2 "Auto-following $name"
 			echo ".${name}:${name}"