scmutil: make shortesthexnodeidprefix() use unfiltered repo
Both callers were doing this, and resolvehexnodeidprefix() was also
working on the unfiltered repo, so it makes more sense to have it all
in one place.
Differential Revision: https://phab.mercurial-scm.org/D3313
#require clang-libfuzzer test-repo
$ cd $TESTDIR/../contrib/fuzz
$ make
Just run the fuzzer for five seconds to verify it works at all.
$ ./bdiff -max_total_time 5