context: convert to hex for error message only for 20-byte changeid
Now that 20-byte strings unambiguously mean binary (or a bug), we can
specialize the conversion to hex for that case.
Differential Revision: https://phab.mercurial-scm.org/D3452
#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