testing: Properly exclude files for linting in Windows.

For files to be excluded in Windows, their paths have to
be normalized, so that the string matching works.

tools/custom_check.py

@@ -161,7 +161,7 @@ def build_custom_checkers(by_lang):
         # This rule might give false positives in virtualenv setup files which should be excluded,
         # and comments which should be rewritten to avoid use of "python2", "python3", etc.
         {'pattern': 'python[23]',
-         'exclude': set(['tools/provision',]),
+         'exclude': set(['tools/provision']),
          'description': 'Explicit python invocations should not include a version'},
         {'pattern': '(^|\s)open\s*\(',
          'description': 'open() should not be used in Zulip\'s bots. Use functions'

tools/server_lib/lister.py

@@ -66,7 +66,7 @@ def list_files(targets=[], ftypes=[], use_shebang=True, modified_only=False,
     # sys.argv as str, so that battle is already lost.  Settle for hoping
     # everything is UTF-8.
     repository_root = subprocess.check_output(['git', 'rev-parse', '--show-toplevel']).strip().decode('utf-8')
-    exclude_abspaths = [os.path.join(repository_root, fpath).rstrip('/') for fpath in exclude]
+    exclude_abspaths = [os.path.normpath(os.path.join(repository_root, fpath)) for fpath in exclude]
 
     cmdline = ['git', 'ls-files'] + targets
     if modified_only: