Fix typo in command to log pip-diff failure

[cjolowicz]

Due to a typo, mount(8) is invoked instead of mcount from heroku/buildpack-stdlib.

The pip-diff tool from vendor/pip-pop is used to determine stale requirements. When pip-diff encounters an unexpected failure, a count should be logged using mcount.

[cjolowicz]

It appears that Travis CI is broken for external pull requests because the environment variables HEROKU_API_USER and HEROKU_API_KEY are secret and thus not available to pull requests from another repository.

/home/travis/build/heroku/heroku-buildpack-python/vendor/bundle/ruby/2.4.0/gems/heroku_hatchet-4.0.6/etc/ci_setup.rb:10:
in `fetch': key not found: "HEROKU_API_USER" (KeyError)

See https://github.com/heroku/hatchet/blob/master/README.md#install

---

You can see the same changes actually passing CI in this PR in my fork:

• Fix typo in command to log pip-diff failure cjolowicz/heroku-buildpack-python#12

This PR is based against my fork, with HEROKU_API_USER and HEROKU_API_KEY linked to my own account, so the Heroku credentials are available to the CI jobs.

I would like to contribute some other changes (see cjolowicz#13 and cjolowicz#14), so I'm wondering how to best make PRs for this repository.

[cjolowicz]

Here is an example for when this bug is hit. pip-diff failed because the cached requirements.txt referenced a file that had been removed. After the traceback, mount is invoked, resulting in an error message (see last line).

Traceback (most recent call last):
  File "/app/.heroku/python/lib/python3.7/site-packages/pip/_internal/download.py", line 665, in get_file_content
    with open(url, 'rb') as f:
FileNotFoundError: [Errno 2] No such file or directory: 'requirements/base.txt'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/app/tmp/buildpacks/33e977e2b813adb8f433f2a7adcfb3ddb2a56e566732d9bbea567c8a3d9f23de56cc92a95c5eceecf6c023272911ba623f1932889b0b52ecd7f577446edfa659/vendor/pip-pop/pip-diff", line 132, in <module>
    main()
  File "/app/tmp/buildpacks/33e977e2b813adb8f433f2a7adcfb3ddb2a56e566732d9bbea567c8a3d9f23de56cc92a95c5eceecf6c023272911ba623f1932889b0b52ecd7f577446edfa659/vendor/pip-pop/pip-diff", line 128, in main
    diff(**kwargs)
  File "/app/tmp/buildpacks/33e977e2b813adb8f433f2a7adcfb3ddb2a56e566732d9bbea567c8a3d9f23de56cc92a95c5eceecf6c023272911ba623f1932889b0b52ecd7f577446edfa659/vendor/pip-pop/pip-diff", line 100, in diff
    r1 = Requirements(r1)
  File "/app/tmp/buildpacks/33e977e2b813adb8f433f2a7adcfb3ddb2a56e566732d9bbea567c8a3d9f23de56cc92a95c5eceecf6c023272911ba623f1932889b0b52ecd7f577446edfa659/vendor/pip-pop/pip-diff", line 44, in __init__
    self.load(reqfile)
  File "/app/tmp/buildpacks/33e977e2b813adb8f433f2a7adcfb3ddb2a56e566732d9bbea567c8a3d9f23de56cc92a95c5eceecf6c023272911ba623f1932889b0b52ecd7f577446edfa659/vendor/pip-pop/pip-diff", line 54, in load
    for requirement in parse_requirements(reqfile, finder=finder, session=requests):
  File "/app/.heroku/python/lib/python3.7/site-packages/pip/_internal/req/req_file.py", line 113, in parse_requirements
    for req in req_iter:
  File "/app/.heroku/python/lib/python3.7/site-packages/pip/_internal/req/req_file.py", line 233, in process_line
    for req in parsed_reqs:
  File "/app/.heroku/python/lib/python3.7/site-packages/pip/_internal/req/req_file.py", line 104, in parse_requirements
    filename, comes_from=comes_from, session=session
  File "/app/.heroku/python/lib/python3.7/site-packages/pip/_internal/download.py", line 669, in get_file_content
    'Could not open requirements file: %s' % str(exc)
pip._internal.exceptions.InstallationError: Could not open requirements file: [Errno 2] No such file or directory: 'requirements/base.txt'
mount: failure.bad-requirements: No such file or directory

[cjolowicz]

Rebased onto master (v157).