narrow: mark the critical chunks of narrowing/widening as unsafe
I'm _mostly_ sure these are the only unsafe chunks here.
Differential Revision: https://phab.mercurial-scm.org/D3718
repair: mark the critical section of strip() as unsafe
When our experimental nointerrupt feature is enabled, this will
help prevent users from corrupting their repo during a strip.
Differential Revision: https://phab.mercurial-scm.org/D3717