buildman: Try to guess the upstream commit

Buildman normally obtains the upstream commit by asking git. Provided that
the branch was created with 'git checkout -b <branch> <some_upstream>' then
this normally works.

When there is no upstream, we can try to guess one, by looking up through
the commits until we find a branch. Add a function to try this and print
a warning if buildman ends up relying on it.

Also update the documentation to match.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Wolfgang Denk <wd@denx.de>