[BUG] rebase should desambiguate abbreviated hashes before starting

I just ran into a funny edge-case when doing a long rebase: one of
the rewritten commits got a sha1 starting with one of the abbreviated
sha1's of a commit still to be applied.

As a result, the rebase stopped with a funny-looking "short SHA1 ... was
ambiguous", which would not have occured if the shortened sha1's presented
to the user were expanded to full sha1's before starting the rebase.

-- 
Yann Dirson - Bertin Technologies