workflows/check-cherry-picks: reduce checkout time

Using a `tree:0` filter instead of `blob:none` reduces the checkout time
from over 3 minutes to about 45 seconds. The required trees/blobs will
then be fetched on-demand.

This on-demand fetching creates additional output for `git range-diff`
on stderr, so we hide that. This only happens the first time it's run,
so we don't need to adjust the other calls - which will still return any
real errors, should they happen.

.github/workflows/check-cherry-picks.yml

@@ -20,7 +20,7 @@ jobs:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
           fetch-depth: 0
-          filter: blob:none
+          filter: tree:0
           path: trusted
 
       - name: Check cherry-picks

ci/check-cherry-picks.sh

@@ -59,7 +59,7 @@ while read -r new_commit_sha ; do
           '"$new_commit_sha~..$new_commit_sha"'
         '
 
-        if $range_diff_common --no-color | grep -E '^ {4}[+-]{2}' > /dev/null ; then
+        if $range_diff_common --no-color 2> /dev/null | grep -E '^ {4}[+-]{2}' > /dev/null ; then
           if [ "$GITHUB_ACTIONS" = 'true' ] ; then
             echo ::endgroup::
             echo -n "::warning ::"