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This enables using the environment variable `SERVO_SALTFS_ROOT` to use a local copy of the saltfs tree for `./mach bootstrap`. The immediate use case is to add a test for `./mach bootstrap` to saltfs itself, to ensure Salt changes or refactors don't break `./mach bootstrap`.
I tested this locally in an Ubuntu Trusty VM.
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- [ ] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).
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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because the test will be added to saltfs
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Source-Repo: https://github.com/servo/servo
Source-Revision: eb058e20a91b1646d0ac1176d4084b14b7c02fc2
a3273f9c10