fuzz: clean up some repetition on building parsers.so fuzzers
There was a lot of repetition here that I realized could be cleared out. I
think there's some more work we could do here, but this is enough for now.
Differential Revision: https://phab.mercurial-scm.org/D7568
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[255]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate