Exclude default config value from the pip-compile header

[atugushev]

Resolves #1863 (comment)

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[chrysle]

Looks like something is wrong with QA & CI.. Let's restart the jobs tomorrow. And maybe wait until #1894 is merged for the failing PyPy tests.

[webknjaz]

This will need rebasing on top of #1895.

[q0w]

@atugushev There is a bug with pip 6.14.0.
I just use pip-compile --allow-unsafe --generate-hashes --output-file=requirements/test.txt --resolver=backtracking --strip-extras requirements/test.in
but now it adds --config=pyproject.toml even there is no configuration in pyproject.toml
When running pip-compile click context has no src_files, so select_config_file returns pyproject.toml, but after compiling click context has src_files as requirements/test.in, so select_config_file returns None and so pyproject.toml != None

Making --config is_eager=False works
#1902

[atugushev]

@q0w thanks for heads up. Referenced this in #1903