tracing: new module to make tracing events in hg easier
This lives in hgdemandimport because I want to instrument a bunch of
low-level stuff including the bare `hg` script and demandimport, so it
can't live at a higher layer.
Differential Revision: https://phab.mercurial-scm.org/D4344
cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=python "$TESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF