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 rev: 9b7243a27702da4e80b2ac92f9adc4ccbf9cf816)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>