Switch to consuming JSON from GitHub for "latest commit touching file"

For some reason, https://github.com/xxx/yyy/commits/HEAD/zzz.atom has recently changed from returning XML by default to returning JSON, which from https://docs.github.com/en/rest/activity/feeds?apiVersion=2022-11-28 seems like it's intentional, so this adds an explicit `Accept:` header (the only explicit JSON one that seems to work) and uses `jq` to determine the latest `hack/dind` commit instead of hacking up the XML via `grep`+`awk`.

See also docker-library/docker#472

.libs/git.sh

@@ -65,9 +65,9 @@ github-file-commit() {
 	[ -n "$file" ]
 
 	local atom
-	atom="$(wget -qO- "https://github.com/$repo/commits/$branch/$file.atom")"
+	atom="$(wget -qO- --header 'Accept: application/json' "https://github.com/$repo/commits/$branch/$file.atom")"
 	local commit
-	commit="$(awk <<<"$atom" -F ' *[<>/]+' '$2 == "id" && $3 ~ /Commit/ { print $4; exit }')"
+	commit="$(jq <<<"$atom" -r '.payload | first(.commitGroups[].commits[].oid)')"
 
 	echo >&2 "github $repo - $file: $commit"