Git support for --changedFilesWithAncestor

[alsuren]

Summary

as requested in #5188, this PR adds git support for the --changedFilesWithAncestor flag.

Test plan

If this is working, you should be able to do this in the jest repo:
yarn jest -- --onlyChanged --changedFilesWithAncestor

[codecov-io]

Codecov Report

Merging #5189 into master will increase coverage by 0.04%.
The diff coverage is 95.65%.

@@            Coverage Diff             @@
##           master    #5189      +/-   ##
==========================================
+ Coverage   60.54%   60.59%   +0.04%     
==========================================
  Files         201      201              
  Lines        6707     6712       +5     
  Branches        4        4              
==========================================
+ Hits         4061     4067       +6     
+ Misses       2645     2644       -1     
  Partials        1        1

Impacted Files	Coverage Δ
packages/jest-changed-files/src/git.js	93.93% <95.65%> (+4.65%)	⬆️

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[SimenB]

Thanks

[SimenB]

Missing docs update, but we still don't have versioned docs, so not sure how to handle it

We don't seem to document it at all

[SimenB]

Would you mind adding a note in the docs about this option? https://github.com/facebook/jest/blob/master/docs/CLI.md

[alsuren]

I noticed that both --lastCommit and --changedFilesWithAncestor are ignored if you don't also pass --onlyChanged or --watch. I thought about writing something in the check() function (line 15 in this file) explode if you pass an argument that will be ignored, but then I realised that someone might set --onlyChanged or --watch in their configs, or someone might add another arg that has the same effect, so I just decided to document it a bit harder.

I also decided against talking about --watch in the docs for --lastCommit, because it sounds like a pointless thing to do.

[SimenB]

I noticed that both --lastCommit and --changedFilesWithAncestor are ignored if you don't also pass --onlyChanged or --watch.

That doesn't seem right. @cpojer WDYT?

[alsuren]

$ yarn jest --listTests | wc -l
     213
$ yarn jest --listTests --lastCommit | wc -l
     213
$ yarn jest --listTests --lastCommit --onlyChanged | wc -l
       4

@SimenB are you suggesting that:
a) we should make --lastCommit and --changedFilesWithAncestor imply --onlyChanged (may cause previously valid configurations to test fewer things than they used to)
b) we should make --lastCommit and --changedFilesWithAncestor explode if it's not combined with --onlyChanged or --watch (may cause previously valid configurations to explode, and means that the next person to add an --onlyChanged synonym needs to update the check)

or are you just after a second reviewer?

[SimenB]

or are you just after a second reviewer?

This 🙂

[cpojer]

Oh, interesting. I think the reason for that is because the default at FB is that onlyChanged is true and people must pass explicitly --all to run all tests. I guess we just never noticed the inconsistency. I think --lastCommit --all doesn't make any sense, so I'm in favor of the proposed change a).

[cpojer]

This is awesome, thanks so much for adding this feature!

[github-actions]

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