CONTRIBUTING

run-tests: support per-line conditional output in tests

Duplicating entire tests just because the output is different is both error
prone and can make the tests harder to read.  This harnesses the existing '(?)'
infrastructure, both to improve readability, and because it seemed like the path
of least resistance.

The form is:

  $ test_cmd
  output (hghave-feature !) # required if hghave.has_feature(), else optional
  out2 (no-hghave-feature2 !) # req if not hghave.has_feature2(), else optional

I originally extended the '(?)' syntax.  For example, this:

  2 r4/.hg/cache/checkisexec (execbit ?)

pretty naturally reads as "checkisexec, if execbit".  In some ways though, this
inverts the meaning of '?'.  For '(?)', the line is purely optional.  In the
example, it is mandatory iff execbit.  Otherwise, it is carried forward as
optional, to preserve the test output.  I tried it the other way, (listing
'no-exec' in the example), but that is too confusing to read.  Kostia suggested
using '!', and that seems fine.

Our full contribution guidelines are in our wiki, please see:

https://www.mercurial-scm.org/wiki/ContributingChanges

If you just want a checklist to follow, you can go straight to

https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist

If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).