tests/test-execute-bit.t

tests: add a test runner utility that prints nothing when all tests pass

This will be used to run tests through run-tests, which will expect no output
for a unit test that passes successfully.

The motivation for using unit tests instead of the current Python tests is that
they don't require an output file for comparison and that they're easier to write
because of the available tools from unittest (setup, asserts).

  $ "$TESTDIR/hghave" execbit || exit 80

  $ 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