run-tests.py: make tests use same python interpreter as test harness.

this is wanted because some tests run python interpreter directly.

must use same python interpreter in tests as in main harness or problems
will happen because of e.g. different python abi if run-tests.py run
with python 2.5 but system python is 2.4.

fix is to see if system python is used and is named python.  if no,
put symlink called python at front of shell search path.

tests/run-tests.py

@@ -79,6 +79,23 b' def cleanup_exit():'
         print "# Cleaning up HGTMP", HGTMP
     shutil.rmtree(HGTMP, True)
 
+def use_correct_python():
+    # some tests run python interpreter. they must use same
+    # interpreter we use or bad things will happen.
+    exedir, exename = os.path.split(sys.executable)
+    if exename == 'python':
+        path = find_program('python')
+        if os.path.dirname(path) == exedir:
+            return
+    vlog('# Making python executable in test path use correct Python')
+    my_python = os.path.join(BINDIR, 'python')
+    try:
+        os.symlink(sys.executable, my_python)
+    except AttributeError:
+        # windows fallback
+        shutil.copyfile(sys.executable, my_python)
+        shutil.copymode(sys.executable, my_python)
+            
 def install_hg():
     vlog("# Performing temporary installation of HG")
     installerrs = os.path.join("tests", "install.err")
@@ -102,6 +119,8 b' def install_hg():'
     os.environ["PATH"] = "%s%s%s" % (BINDIR, os.pathsep, os.environ["PATH"])
     os.environ["PYTHONPATH"] = PYTHONDIR
 
+    use_correct_python()
+
     if coverage:
         vlog("# Installing coverage wrapper")
         os.environ['COVERAGE_FILE'] = COVERAGE_FILE