tests/test-issue4074.t

revlog: fix pure version of _partialmatch() to include nullid

Before this patch, test-issue842.t and a few more tests fail when they
try to refer to the null revision by using a "000.." prefix of it (or
because they use the "shortest" template function which internally
does that).

This should have been part of my a3dacabd476b (index: don't allow
index[len(index)] to mean nullid, 2018-07-20), but I had forgotten to
update another part of the pure code there, so it didn't fail until
a1f934573c0b (parsers: adjust pure-python version to mimic
a3dacabd476b, 2018-08-09) and 65d5de1169dd (revlog: fix pure nodemap
to not access missing index entry, 2018-08-17) fixed the other things
I had missed.

Differential Revision: https://phab.mercurial-scm.org/D4332

#require no-pure

A script to generate nasty diff worst-case scenarios:

  $ cat > s.py <<EOF
  > import random
  > for x in range(100000):
  >     print
  >     if random.randint(0, 100) >= 50:
  >         x += 1
  >     print(hex(x))
  > EOF

  $ hg init a
  $ cd a

Check in a big file:

  $ $PYTHON ../s.py > a
  $ hg ci -qAm0

Modify it:

  $ $PYTHON ../s.py > a

Time a check-in, should never take more than 10 seconds user time:

  $ hg ci --time -m1
  time: real .* secs .user [0-9][.].* sys .* (re)