Log a warning if the user specifies -P and the output file is present but empty

[davidmreed]

When the user runs a command like

pip-compile -P some_package requirements.in

the expected behavior is that only some_package should be updated to a new version.

If the user errs by redirecting pip-compile's stdout to the target file:

pip-compile -P some_package requirements.in > requirements.txt

instead of either using -o or allowing pip-compile to infer the target file name, the target file is truncated by the shell before pip-compile executes. This results in pip-compile actually updating all dependencies, because while the target file is present, it's not a valid source of version constraints.

This PR adds a check in the code to detect this situation, which is almost certainly undesired, and redirect the user to a more appropriate command.

Contributor checklist

• Provided the tests for the changes.
• Assure PR title is short, clear, and good to be included in the user-oriented changelog

Maintainer checklist

• Assure one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[chrysle]

LGTM, but I'd like another person to proof-read this.

[webknjaz]

@chrysle no wording suggestions from me, but I've got a few nitpicks on the engineering side of things… Though, they are not mandatory, and it's okay to merge if @davidmreed chooses to leave things as is.