The towncrier step fails sometimes

[exarkun]

For reasons unknown:

Traceback (most recent call last):
  File "/opt/python/2.7.14/lib/python2.7/runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/opt/python/2.7.14/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/home/travis/build/LeastAuthority/tahoe-lafs/.tox/codechecks/lib/python2.7/site-packages/towncrier/check.py", line 86, in <module>
    _main()
  File "/home/travis/build/LeastAuthority/tahoe-lafs/.tox/codechecks/lib/python2.7/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/home/travis/build/LeastAuthority/tahoe-lafs/.tox/codechecks/lib/python2.7/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/home/travis/build/LeastAuthority/tahoe-lafs/.tox/codechecks/lib/python2.7/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/travis/build/LeastAuthority/tahoe-lafs/.tox/codechecks/lib/python2.7/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/home/travis/build/LeastAuthority/tahoe-lafs/.tox/codechecks/lib/python2.7/site-packages/towncrier/check.py", line 26, in _main
    return __main(compare_with, directory)
  File "/home/travis/build/LeastAuthority/tahoe-lafs/.tox/codechecks/lib/python2.7/site-packages/towncrier/check.py", line 35, in __main
    _run(["git", "diff", "--name-only", comparewith + "..."], cwd=base_directory)
  File "/home/travis/build/LeastAuthority/tahoe-lafs/.tox/codechecks/lib/python2.7/site-packages/towncrier/check.py", line 19, in _run
    return check_output(args, **kwargs)
  File "/opt/python/2.7.14/lib/python2.7/subprocess.py", line 219, in check_output
    raise CalledProcessError(retcode, cmd, output=output)
subprocess.CalledProcessError: Command '['git', 'diff', '--name-only', u'origin/master...']' returned non-zero exit status 128
ERROR: InvocationError for command '/home/travis/build/LeastAuthority/tahoe-lafs/.tox/codechecks/bin/python -m towncrier.check' (exited with code 1)

Only in certain build environments.

[exarkun]

This seems to be a combination of TravisCI's behavior on "branch builds":

git clone --depth=1000 --branch=towncrier-lint-failures https://github.com/LeastAuthority/tahoe-lafs.git

which results in a masterless checkout and towncrier's method of implementing its check feature:

['git', 'diff', '--name-only', u'origin/master...']

The problem is somewhat exacerbated by the towncrier behavior of hiding the git output in this case.

I'm going to call this not a bug in Tahoe-LAFS. I've disabled "branch build" for LeastAuthority?/tahoe-lafs (not even sure why it was on) and sent a PR to towncrier to improve error reporting (​https://github.com/hawkowl/towncrier/pull/125).

[exarkun]

Branch builds are still enabled for the tahoe-lafs/tahoe-lafs repo, of course. Can I disable those? Hmmm.

[exarkun]

I disabled them. If it's a problem, we can re-enable them and try something else.