subrepo: add basic support to hgsubrepo for the files command
Paths into the subrepo are not yet supported.
The need to use the workingctx in the subrepo will likely be used more in the
future, with the proposed working directory revset symbol. It is also needed
with archive, if that code is to be reused to support 'extdiff -S'.
Unfortunately, it doesn't seem possible to put the smarts in subrepo.subrepo(),
as it breaks various status and diff tests.
I opted not to pass the desired revision into the subrepo method explicitly,
because the only ones that do pass an explicit revision are methods like status
and diff, which actually operate on two contexts- the subrepo state and the
explicitly passed revision.
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