run-tests: report code coverage from source directory

As part of testing code coverage output, I noticed some files were
being reported twice: there was an entry for the file in the install
location and for the file in the source tree. I'm not sure why this
is. But it resulted in under-reporting of coverage data since some
lines weren't getting covered in both locations.

I also noticed that files in the source directory and outside the
"mercurial" and "hgext" packages were getting included in the
coverage report. Cosmetically, this seemed odd to me. It's not
difficult to filter paths from the report. But I figure this data
can be useful (we could start reporting run-tests.py coverage,
for example).

This patch switches the coverage API to report code coverage from
the source directory. It registers a path alias so that data from
the install location is merged into data from the source directory.
We now get merged results for files that were being reported in
multiple locations.

Since code coverage reporting now relies on the profiled install
now being in sync with the source tree, an additional check to
disallow code coverage when --with-hg is specified has been added.
This should have been present before, as --local was previously
disallowed for the same reasons.

Merging the paths raises our aggregate line coverage from ~60 to
81%.

tests/run-tests.py

@@ -260,6 +260,10 b' def parseargs(args, parser):'
         parser.error("sorry, coverage options do not work when --local "
                      "is specified")
 
+    if options.anycoverage and options.with_hg:
+        parser.error("sorry, coverage options do not work when --with-hg "
+                     "is specified")
+
     global verbose
     if options.verbose:
         verbose = ''
@@ -1567,6 +1571,7 b' class TestRunner(object):'
 
     def __init__(self):
         self.options = None
+        self._hgroot = None
         self._testdir = None
         self._hgtmp = None
         self._installdir = None
@@ -1872,6 +1877,7 b' class TestRunner(object):'
         # Run installer in hg root
         script = os.path.realpath(sys.argv[0])
         hgroot = os.path.dirname(os.path.dirname(script))
+        self._hgroot = hgroot
         os.chdir(hgroot)
         nohome = '--home=""'
         if os.name == 'nt':
@@ -1996,9 +2002,13 b' class TestRunner(object):'
         vlog('# Producing coverage report')
         # chdir is the easiest way to get short, relative paths in the
         # output.
-        os.chdir(self._pythondir)
+        os.chdir(self._hgroot)
         covdir = os.path.join(self._installdir, '..', 'coverage')
         cov = coverage(data_file=os.path.join(covdir, 'cov'))
+
+        # Map install directory paths back to source directory.
+        cov.config.paths['srcdir'] = ['.', self._pythondir]
+
         cov.combine()
 
         omit = [os.path.join(x, '*') for x in [self._bindir, self._testdir]]