Filter out repositories with .batchignore at root

[mrnugget]

This implements https://github.com/sourcegraph/sourcegraph/issues/18330 by querying the locations of .batchignore files in each repository yielded by the on attribute in a batch spec. If locations were found, the repository is ignored.

This can be overwritten by using the -force-override-ignore flag.

Example: given my instance has the following repositories:

• github.com/sourcegraph-testing/zap
• github.com/sourcegraph-testing/titan
• github.com/sourcegraph-testing/tidb
• github.com/sourcegraph-testing/etcd
• github.com/sourcegraph-testing/batch-changes-testing-ignore

And I use the following batch spec:

on:
  - repositoriesMatchingQuery: repohasfile:README.md repo:sourcegraph-testing

steps:
  - run: echo "a horse says 'hello'" >> README.md
    container: alpine:3

with this change the batch-changes-testing-ignore repository will be ignored:

• no archive will be downloaded
• no steps executed

A message is printed that says it's ignored.

[mrnugget]

Closing this for now, since I discussed this with @malomarrec on a Zoom call right now. Outcome: we don't want to exclude those repositories from the search results, because we want to print which repositories were ignored. See https://github.com/sourcegraph/sourcegraph/issues/18330#issuecomment-815752987

[mrnugget]

Turns out that the functionality we built for monorepo support — a method that returns the locations of a file in a list of repositories — can be used here: instead of adding a -repohasfile:.batchignore filter to the search query I changed the code to search the locations of .batchignore files in all the yielded repositories.

screenshot_2021-04-08_15.05.48.mp4

[LawnGnome]

This is surprisingly clean! A real testament to your design work when implementing the file matching in the first place, I think.

This would need documentation and a changelog, of course, but otherwise I think this is good to go.