tests/narrow-library.sh

changegroup: move file matcher from narrow extension

Sparse changegroup generation requires the use of a matcher to
filter which files are relevant.

This commit moves the file matcher from the narrow extension to core
and updates the narrow extension to use it.

I'm not sure why the narrow extension was storing the matcher as
a callable that resolved to a matcher. So I changed it to be a
simple matcher instance.

In addition, code from narrow to intersect the matcher with the local
narrow spec is now performed automatically when the changegroup
packer is created.

If a matcher is not passed into getbundler() an alwaysmatcher() is
assumed. This ensures that a matcher is always defined for all
operations.

Differential Revision: https://phab.mercurial-scm.org/D4011

cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=python "$TESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF