arc: fix performance of "arc unit" with vanilla arcanist
The upstream phacility libphutil code sets the PHP "xdebug.max_nesting_level"
config setting to PHP_INT_MAX. When using HHVM this turns on XDebug profiling,
which appears to make things substantially slower, particularly on Linux.
This was making "arc unit" take substantially longer, and even caused test
failures in some cases. (For instance, some tests check for messages saying
that hg obtained the lock after 1 second, but this can take substantially
longer than 1 second with XDebug profiling enabled.)
Resetting xdebug.max_nesting_level back to 0 turns profiling back off again,
and fixes the performance issues.
Ran arc unit and confirmed that it ran in roughly the same time as running
Reviewers: stash, ryanmce, quark, #fbhgext
Reviewed By: quark, #fbhgext
Differential Revision: https://phab.mercurial-scm.org/D397