tests/test-execute-bit.t

exthelper: reintroduce the ability to register revsets

I think this is what Yuya and Boris agreed on.[1]  This happens *after* the
extsetup phase now (and after the _aftercallback notifications).  But this is
trivial, mergeable between exthelper instances, and doesn't need to have the
extension name supplied when registering.

The test needed updating so that extsetup() takes a `ui` argument, as exthelper
isn't trying to be backward compatible with 1.3.1.

[1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2018-December/125888.html

#require execbit

  $ hg init
  $ echo a > a
  $ hg ci -Am'not executable'
  adding a

  $ chmod +x a
  $ hg ci -m'executable'
  $ hg id
  79abf14474dc tip

Make sure we notice the change of mode if the cached size == -1:

  $ hg rm a
  $ hg revert -r 0 a
  $ hg debugstate
  n   0         -1 unset               a
  $ hg status
  M a

  $ hg up 0
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg id
  d69afc33ff8a
  $ test -x a && echo executable -- bad || echo not executable -- good
  not executable -- good