Add --stash as alternative to --clean to update-checkout

[MaxDesiatov]

Use of --clean can lead to irreversible loss of uncommitted data. It is still desirable to reset Swift project repositories to a clean state, but without deleting all of the in-progress changes. Passing --stash instead of (or in addition to) --clean will preserve uncommitted changes in stashes of corresponding repositories instead of completely deleting those.

[MaxDesiatov]

@swift-ci smoke test

[MaxDesiatov]

@swift-ci smoke test macos

[AnthonyLatsis]

LGTM. A test would be nice.

[MaxDesiatov]

@swift-ci smoke test

[MaxDesiatov]

@swift-ci smoke test

[MaxDesiatov]

LGTM. A test would be nice.

Can you provide some details on what would be the best way to cover this with tests, in your opinion?

[AnthonyLatsis]

update-checkout has a Python unit test suite at utils/update_checkout/tests. You can run it using utils/update_checkout/run_tests.py or lit + validation-test/Python/update_checkout.swift. The test suite is centered around an intermediary unittest.TestCase subclass from which all the effective test classes inherit. This base class is responsible for setting up and tearing down a mock-up environment consisting of a directory of local remotes and a source root — a directory for their clones. Subclasses define test functions that run update-checkout commands against these mock repositories and perform state checks.

I think a single test function that

1. clones the remotes using update-checkout
2. creates some untracked files in several clones
3. runs update-checkout with --stash
4. verifies that the untracked files have indeed been stashed and nothing else happened

would be good enough for now (the update/clone tests, if any, are really basic anyway). I would place this function in a new subclass in its own file.