tests: improve the widening testing in test-narrow-widen*
Before this patch, we are testing `hg tracked --addinclude` by adding a command
which is not introduced in the changesets till now.
If you closely look at the tests, wider/f was introduced on the server after the
narrow clone was done and extending the existing clone to include wider/f does
not make sense. We should test extending a file which exists.
Differential Revision: https://phab.mercurial-scm.org/D4452
narrow: use util.readfile() and improve error message using --narrowspec
This patch improves the error message and uses util.readfile() for reading
narrowspecs file while cloning.
Differential Revision: https://phab.mercurial-scm.org/D4462