rhg: Skip reading the contents of ambiguous files in some cases
If the size of the file in the working directory does not match the length of
the filelog data, we know its contents will be different and don’t need to
read it.
rhg still decodes the filelog revision, which is not needed in some cases.
Differential Revision: https://phab.mercurial-scm.org/D11910
#require docutils gettext
$ $TESTDIR/check-gendoc ru
checking for parse errors