rhg: read [paths] for `--repository` value

hg parses -R and --repository CLI arguments "early" in order to know which
local repository to load config from. (Config can then affect whether or how to
fall back.)

The value of of those arguments can be not only a filesystem path, but also an
alias configured in the [paths] section. This part was missing in rhg and this
patch implements that.

Differential Revision: https://phab.mercurial-scm.org/D10296
14 jobs for topic/default/rust-rocket in 26 minutes and 50 seconds (queued for 5 seconds)
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phabricator-refresh

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