git: Use git branch for ancestor checks

Using git merge-base for checking for ancestors is nice but required git 1.8.0
which is not in many distrbutions yet. We therefore revert to a more ugly
check using git branch --contains until such times as we can upgrade.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
fetch2: Fix handling of SCM mirrors in MIRRORS

If an SCM mirror is in PREMIRRORS, the tarball is downloaded and then found
by the "upstream" check and handled correctly.

If an SCM mirror is in MIRRORS, the tarball is downloaded but not used
since there is no "upstream" run after MIRRORS completes. It therefore
sits there useless and unused. This code change forces the upstream to
run after a mirror tarball is found and fixes the usage of SCM mirrors
in MIRRORS.

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