[test] Fix prettier ci task with multiple changed files

[eps1lon]

Fixes various issues in #12564:

• no changes caused grep to match everything (this is a non-issue because why would this trigger a CI build?)
• more than one changed file broke the if statement (I'm concerned that an error in a bash command does not cause the CI task to fail)
• passing more than one file arg to the cli was considered on file

Sorry about the inconvenience. I should've been more careful with languages that I'm not that familiar with.

The build should be considered broken. Once this is confirmed I will retrigger with only one file changed and the remove the broken format completely. Only if all those builds return as expected should this be merged.

[eps1lon]

This does work locally git diff --name-only master... returns different results in CI. I'm going to investigate how I can diff against master in CircleCI.

[eps1lon]

The current approach relies on CIRCLE_COMPARE_URL which is empty at the moment. No idea why (it was set on previous builds).

I have no idea how to get a commit range from master to branch in CircleCI otherwise. #12564 should be reverted if no one else has solution.

[oliviertassinari]

I have no idea how to get a commit range from master to branch in CircleCI

The test_material-ui-x test can help.

[eps1lon]

Thanks for the hint @oliviertassinari. Responsible for the unexpected git diff master behavior is the hard reset that CircleCI does when checking out the code.

I had to work around CircleCIs missing support for interpolation while setting environment variables quite a bit.

The prettier step now exited as expected on the following test case:

• no changed files considered by prettier
• one changed file considered by prettier
• two changed files considered by prettier

[oliviertassinari]

Thanks. I wish we had a detailed diff of what's wrong (like before) rather than just the files that need to be fixed, but it's not that important (easier to graspe for a new contributor). Let's try that out.

[eps1lon]

While this is useful for linting since most of those issues need to be resolved manually formatting should not be done manually. That should either be handled by the IDE or via yarn prettier.