rhg: do not try to open a nodemap for an inline index
This saves an [open] system call per file, which is a small saving, but
it showed up in the profile at large file counts (it accounted for 30ms
out of 400ms needed for catting 10000 files, on a ZFS filesystem on Linux,
so ~3us per syscall).
Differential Revision: https://phab.mercurial-scm.org/D11659
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