Auto merge of #19550 - kregoslup:issue_15725, r=jdm

Issue #15723: Skip git ignored files in update-manifest.

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This patch makes "./mach update-manifest" aware of .gitignore.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15725 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because I don't know how to test it, I have tried mocking opening file and checking whether it exists but it's difficult to handle both in py2.7 and py3 and I think it would say too much about `PathFilter` internals. I have also tried mocking `PathFilter` but then I would be duplicating logic in tests.

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tests/wpt/mozilla/.gitignore

@@ -0,0 +1,10 @@
+*#
+*.py[co]
+*.sw[po]
+*~
+MANIFEST.json
+\#*
+.vscode/
+.DS_Store
+*.rej
+_venv