patchbomb: allow using HGHOSTNAME to force a hostname
I'll update run-tests.py to set this globally to stabilize some
tests. The variable name is intentionally generic because I suspect we
should generalize this to other tests.
Differential Revision: https://phab.mercurial-scm.org/D4294
rebase: cover restorestatus() by lock to prevent it from being updated
To prevent it from being updated by another process
`restorestatus()` is moved under lock.
Differential Revision: https://phab.mercurial-scm.org/D4282