remotenames: add functions to read remotenames data from .hg/remotenames/
This patch functions which can be used to read remotenames data from
.hg/remotenames/. The logic for the function which reads the remotenames file is
taken from the remotenames extension.
Previously reviewed as D940.
Differential Revision: https://phab.mercurial-scm.org/D1550
test-convert-git.t
test-subrepo-git.t
test-patchbomb-tls.t