setup/git.sh: parser GIT_COMMITTER_DATE with plumbing command

To avoid:
- Commit's signature
- Future change in git log format
- Revision Walk

Close #33165

common/environment/setup/git.sh

@@ -17,7 +17,8 @@ elif [ -z "${SOURCE_DATE_EPOCH}" ]; then
 	if [ -n "$basepkg" -a -z "$($XBPS_GIT_CMD -C ${XBPS_SRCPKGDIR}/${basepkg} ls-files template)" ]; then
 		export SOURCE_DATE_EPOCH="$(stat -c %Y ${XBPS_SRCPKGDIR}/${basepkg}/template)"
 	else
-		export SOURCE_DATE_EPOCH="$($XBPS_GIT_CMD -C ${XBPS_DISTDIR} log --pretty='%ct' -n1 HEAD)"
+		export SOURCE_DATE_EPOCH=$($XBPS_GIT_CMD cat-file commit HEAD |
+			sed -n '/^committer /{s/.*> \([0-9][0-9]*\) [-+][0-9].*/\1/p;q}')
 	fi
 fi