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
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a