[StGIT RFC PATCH 0/2] Fixing the rebase safeguard

This is an attempt at using a reachability test to replace the use of orig-base, to decide when it is safe to accept a rebase.

This implementation passes the testcase posted earlier by Karl (resent here as 2nd patch in this series), BUT fails the 4th test of t2100. That is, it fails to deal with the case of a rewinding commit occuring in the upstream branch, and being first git-fetch'd before being stg-pull'd in fetch-rebase mode. In this case, the former upstream commit will really be lost.

In this case, however, it is exactly what we want, but I'm still undecided about how to deal with this best. Possibly insane ideas for now include:

- parsing the remote.*.fetch lines to detect the leading + (just
  kidding ;)
- using ORIG_HEAD, but then, how do we decide when it is valid to do
  so ?

I'll now switch to some real-life activity to see if that helps to find a better solution.