fetch2/git: enhance the unpack by using "git checkout"

fetch2/git: enhance the unpack by using "git checkout"

current git fetcher unpack method only checkout index and working tree,
but did not did not update the git branch in ref/heads, so user may not
get right info in ${S} by using git.

this patch enhance the unpack by using git checkout to fix this issue.

Fix bug [YOCTO #1089]

Signed-off-by: Yu Ke <ke.yu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>