Rebase to parent branch after git-fetch in "stg pull".

Previously we were just assuming that the remote from which we
just failed defined a local branch whose name was the same as the
remote def, and that this branch was the parent.  While this is true
for the most common case (branch "origin" from remote "origin"), it is
quite an unflexible assumption.

Also move the rebasing from git.pull to commands.pull.func since that
is clearly stgit-level stuff, and rename git.pull() to git.fetch() to
match default behaviour.

Signed-off-by: Yann Dirson <ydirson@altern.org>