fetch: Auto-following tags should check connectivity, not existence

[Johannes Sixt <J.Sixt@eudaptics.com>]

git-fetch's auto-following of tags fetches all tags for which it finds
objects in the local repository. I feel it were better if not object
existence, but connectivity to the existing refs was checked, like this:

diff --git a/git-fetch.sh b/git-fetch.sh
index fd70696..1b3c459 100755
--- a/git-fetch.sh
+++ b/git-fetch.sh
@@ -314,11 +314,12 @@ case "$no_tags$tags" in
        taglist=$(IFS=' ' &&
        echo "$ls_remote_result" |
        git-show-ref --exclude-existing=refs/tags/ |
        while read sha1 name
        do
-           git-cat-file -t "$sha1" >/dev/null 2>&1 || continue
+           t=$(git-rev-list --max-count=1 "$sha1" --not --all 2>
/dev/null) &&
+           test -z "$t" || continue
            echo >&2 "Auto-following $name"
            echo ".${name}:${name}"
        done)
    esac
    case "$taglist" in

Is this considered in the shell-to-C rewrite that's currently going on?

The background is that I sometimes fetch a branch with tags, but then
decide to remove it (and the tags as well). git-gc and git-prune do not
guarantee that the objects go away because they could be reachable from
reflogs. Now the next git-fetch will fetch the tags again even though I
do not have any refs from which they would be reachable.

-- Hannes