.arcconfig

merge-lists: make it possible to specify pattern to match

The `merge-lists` tool doesn't know anything about Python other than
its regex that attempts to match import lines. Let's make it possible
to pass in a custom regex so it's easy to use the tool for e.g. C/C++
`#include` lines or Rust `use` lines (given the limited).

Differential Revision: https://phab.mercurial-scm.org/D12392

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