.arcconfig

rhg: add exit code to HgError::Abort()

My previous attempts to have rhg end with correct exit code was more of bug
hunting. I found cases which were failing and fixed them. But as one might
expect, more tests started failing.

Let's add exit code `HgError::Abort()` and make it users explicitly tell what
exit code they want.

Differential Revision: https://phab.mercurial-scm.org/D10838

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