Get the tox coverage environment to use coverage_enable_subprocess

Note `--branch` removed because it's now in the config file, where it needs to
be to be respected by subprocesses.

tox.ini

@@ -54,6 +54,15 @@ commands =
 	 py.test -v integration/
 
 [testenv:coverage]
+deps =
+     {[testenv]deps}
+     coverage_enable_subprocess==1.0
+
+setenv =
+       # Point at the coveragerc file so that any Python child processes we
+       # launch can find it and do the kind of coverage measurement we want.
+       COVERAGE_PROCESS_START={env:PWD}/.coveragerc
+
 # coverage (with --branch) takes about 65% longer to run
 commands =
 	 # As an aid to debugging, dump all of the Python packages and their
@@ -63,7 +72,7 @@ commands =
 	 # somewhat.
          pip freeze
          tahoe --version
-         coverage run --branch -m twisted.trial {env:TAHOE_LAFS_TRIAL_ARGS:--rterrors --reporter=timing} {posargs:allmydata}
+	 coverage run -m twisted.trial {env:TAHOE_LAFS_TRIAL_ARGS:--rterrors --reporter=timing} {posargs:allmydata}
          coverage xml
 
 [testenv:codechecks]