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.

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 ::"