help: prepend environment variables used in hooks with "$"
Most of them have "$" prefix but some don't, so adding it for consistency.
Differential Revision: https://phab.mercurial-scm.org/D10398
BasedOnStyle: LLVM
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IncludeCategories:
- Regex: '^<'
Priority: 1
- Regex: '^"'
Priority: 2