tests/test-issue842.t

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.)

http://mercurial.selenic.com/bts/issue842

  $ hg init
  $ echo foo > a
  $ hg ci -Ama
  adding a

  $ hg up -r0000
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

  $ echo bar > a

Should issue new head warning:

  $ hg ci -Amb
  adding a
  created new head

  $ hg up -r0000
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

  $ echo stuffy > a

Should not issue new head warning:

  $ hg ci -q -Amc

  $ hg up -r0000
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

  $ echo crap > a
  $ hg branch testing
  marked working directory as branch testing
  (branches are permanent and global, did you want a bookmark?)

Should not issue warning:

  $ hg ci -q -Amd