filter-branch: work correctly with ambiguous refnames

filter-branch: work correctly with ambiguous refnames

'git-filter-branch branch' could fail producing the error:
"Which ref do you want to rewrite?" if existed another branch
or tag, which name was 'branch-something' or 'something/branch'.

[jc: original report and fix were done between Dmitry Potapov
and Dscho; I rewrote it using "rev-parse --symbolic-full-name"]

Signed-off-by: Junio C Hamano <gitster@pobox.com>