rebase: use reflog to find common base with upstream

Commit 15a147e (rebase: use @{upstream} if no upstream specified,
2011-02-09) says:

	Make it default to 'git rebase @{upstream}'. That is also what
	'git pull [--rebase]' defaults to, so it only makes sense that
	'git rebase' defaults to the same thing.

but that isn't actually the case.  Since commit d44e712 (pull: support
rebased upstream + fetch + pull --rebase, 2009-07-19), pull has actually
chosen the most recent reflog entry which is an ancestor of the current
branch if it can find one.

Add a '--fork-point' argument to git-rebase that can be used to trigger
this behaviour.  This option is turned on by default if no non-option
arguments are specified on the command line, otherwise we treat an
upstream specified on the command-line literally.

Signed-off-by: John Keeping <john@keeping.me.uk>