tests/blacklists/README

scmutil: consistently return subrepos relative to ctx1 from itersubrepos()

Previously, if a subrepo was added in ctx2 and then compared to another without
it (ctx1), the subrepo for ctx2 was returned amongst all of the ctx1 based
subrepos, since no subrepo exists in ctx1 to replace it in the 'subpaths' dict.
The two callers of this, basectx.status() and cmdutil.diffordiffstat(), both
compare the yielded subrepo against ctx2, and thus saw no changes when ctx2's
subrepo was returned.  The tests here previously didn't mention 's/a' for the
'p1()' case.

This appears to have been a known issue, because some diffordiffstat() comments
mention that the subpath disappeared, and "the best we can do is ignore it".  I
originally ran into the issue with some custom convert code to flatten a tree of
subrepos causing hg.putcommit() to abort, but this new behavior seems like the
correct status and diff behavior regardless.  (The abort in convert isn't
something users will see, because convert doesn't currently support subrepos in
the official repo.)

Put here definitions of blacklists for run-tests.py

Create a file per blacklist. Each file should list the names of tests that you
want to be skipped.
File names are meant to be used as targets for run-tests.py --blacklist
option.
Lines starting with # are ignored. White spaces are stripped.

e.g. if you create a blacklist/example file containing:
 test-hgrc
 # some comment
 test-help
then calling "run-tests.py --blacklist blacklists/example" will exclude
test-hgrc and test-help from the list of tests to run.