config.mak: add extra warning flags

These match what Junio uses for test builds, and can catch
some real-world problems.
config.mak: test GIT_DAEMON by default

There's no reason not to.
rebase: add rebase-on-upstream flag

We use the magic "try to rebase on the last reflog ancestor"
trick that "git-pull --rebase" uses. However, this can go
bad in certain situations (e.g., upstream rewinds and then
goes forward, and the most recent reflog is the "old" one,
making it look like you have a bunch of junk on your
branch).

This option gives an easy way out of it by having a simple
and stupid mode.