feeds: enable cloning of specific git commit

Following syntax should be used to enable this feature:

src-git custom git://openwrt/custom-feed^SHA_COMMIT_ID

Signed-off-by: Luka Perkov <luka@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38172 3c298f89-4303-0410-b956-a3cf2f4a3e73