.arcconfig

fix: ignore fixer tool configurations that are missing patterns

This is to prevent a crash under the same circumstances.

This is also to avoid data loss due to accidental application of a fixer tool
to all files, if the matching logic somehow changed to that effect. Affecting
all files until otherwise configured would be dangerous, and not very useful.

We shouldn't abort because there may be other fixers, and it may still be
useful to run them without having to adjust configuration. A user might not
feel confident in changing configs, for example.

Differential Revision: https://phab.mercurial-scm.org/D6693

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