.clang-format

rust-filepatterns: add a Rust implementation of pattern-related utils

This change introduces Rust implementations of two functions related to
pattern handling, all located in `match.py`:

    - `_regex`
    - `readpatternfile`

These utils are useful in the long-term effort to improve `hg status`'s
performance using Rust. Experimental work done by Valentin Gatien-Baron
shows very promising improvements, but is too different from the current
Mercurial core code structure to be used "as-is".
This is the first - albeit very small - step towards the code revamp
needed down the line.

Two dependencies were added: `regex` and `lazy_static`. Both of them
will be useful for a majority of the Rust code that will be written,
are well known and maintained either by the Rust core team, or by
very frequent contributors.

Differential Revision: https://phab.mercurial-scm.org/D6271

BasedOnStyle: LLVM
IndentWidth: 8
UseTab: ForIndentation
BreakBeforeBraces: Linux
AllowShortIfStatementsOnASingleLine: false
IndentCaseLabels: false
AllowShortBlocksOnASingleLine: false
AllowShortFunctionsOnASingleLine: false
IncludeCategories:
  - Regex:           '^<'
    Priority:        1
  - Regex:           '^"'
    Priority:        2