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Rebase. Per @epriestley: fixed version; renamed callback; updated
error handling; adjusted messaging(s).

Extend ArcanistLinter directly; forgo some of the awesomeness
of the ExternalLinter.

Thanks for instaresponse. Was just going to ask re: remarkup ;)

Classic: updated a git commit message with # for enumerated list; will fix my diff description momentarily.

@marpaia: @johnclyde and I discussed out-of-band and will be doing some work on this later this week.

Been a while, but it looks like I should just use a less ancient version of hg.

@johnclyde: Going to work on this for a bit, unless you've secretly got diffs out for this already. Please let me know.

@epriestley: The TERM option will probably help me for the time being.

@epriestley: Thanks for the context. I will try a newer version and see if that makes things better.

I guess the title here just focuses on one potential solution; one alternative would be
to try harder to kill the children's group while sticking with SIGKILL.

@durham: Thanks for the comments; I'll take a closer look. For max efficiency, is there a specific minimum version of Mercurial that we support here?