bitbake: fetch2/git: Anchor names when using ls-remote

When specifying tags, they're searched for unanchored so foo/bar could
match:

refs/heads/abc/foo/bar
refs/heads/xyz/foo/bar
refs/heads/foo/bar

This change anchors the expressions so they are based against heads
or tags (or any other base level tree that has been created).

(Bitbake master rev: df2e0972cd1db7abd5ec8b7cb295fb0c42e284a4)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>