fsmonitor: add support for extra `hg debuginstall` data
This might make some things easier to debug, and for default bug
report templates it'll help collect more data from users all at
once. I don't actually need fsmonitor in our bug reports (we don't use
it), but this demonstrates the utility of the preceding patches
without having to add new things to core.
Differential Revision: https://phab.mercurial-scm.org/D6682
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