Ignore links to other branches

[jablko]

Initial checklist

• [x] I read the support docs
• [x] I read the contributing guide
• [x] I agree to follow the code of conduct
• [x] I searched issues and couldn’t find anything (or linked relevant results below)
• [x] If applicable, I’ve added docs and tests

Description of changes

Append GitHost.committish -> urlConfig.prefix, thereby only matching links to the configured branch and ignoring others.

Fixes https://github.com/DefinitelyTyped/DefinitelyTyped/issues/58598

In this instance :point_up:, the falsely-failing link was https://github.com/DefinitelyTyped/DefinitelyTyped/blob/1253faabf5e0d2c5470db6ea87795d7f96fef7e2/types/history/v2/tsconfig.json (it's valid).

The linked file exists in that commit but not in the main branch.

Before this PR, urlConfig.prefix is /DefinitelyTyped/DefinitelyTyped/blob/, which matches every branch/commit. After, it's /DefinitelyTyped/DefinitelyTyped/blob/master/, which only matches the configured branch.

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### Initial checklist

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Thanks, — bb

[codecov-commenter]

Codecov Report

Merging #65 (0937e33) into main (f259b30) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #65   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        12           
  Lines          877       890   +13     
=========================================
+ Hits           877       890   +13     

Impacted Files	Coverage Δ
lib/find/config.js	100.00% <100.00%> (ø)
lib/find/find-references.js	100.00% <100.00%> (ø)

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[wooorm]

I’m still not 100% on whether this is a good idea.

But, particularly, I find it unclear what the implications are, because: a) to enable this feature, a “commitish” has to explicitly be passed, and there’s only one test for that. b) there are no docs

What happens if a SHA is passed instead of a branch? What if the branch was automatically detected (like the repo)?

What about common workflows:

• I’m on the main branch and everything’s tested
• I check out my-feature-branch to do some work
• I refactor a bunch of things, but none of the local links are checked, because none of them go to the branch my-feature-branch?
• The branch is merged, tons of errors occur — seems quite unfortunate?

[jablko]

I see what you mean: I've now added docs covering what happens when the repository option does/doesn't have a branch, and what happens when the repo is automatically detected.

In that workflow :point_up::

• If the repository option has a branch (e.g. https://github.com/remarkjs/remark-validate-links.git#main) the plugin checks links to the main branch, regardless of the current branch (my-feature-branch or main).

  Without this PR it doesn't check anything, because currently urlConfig.prefix is /remarkjs/remark-validate-links#main/blob/ which doesn't match any links.

• If it doesn't have a branch the plugin checks links to all branches as if they pointed to the current branch (same behavior as today).

• The same happens when the repo is automatically detected (same as today).

So in every case, local links are checked.

[jablko]

In the latest commit I split up the test into individual test cases.