Use platform-sensitive filename normalization

The os.path.commonprefix() function only does basic string prefix checking, and
isn't aware of case-insensitive file names, or path separators. Instead, this
switches the filename formatting to use os.path.relpath() for normalizing paths
relative to sys.path.

Fixes #67

test/test_utils.py

@@ -0,0 +1,51 @@
+import pytest
+
+import posixpath
+import ntpath
+
+from flask_debugtoolbar.utils import _relative_paths, _shortest_relative_path
+
+
+@pytest.mark.parametrize('value,paths,expected,path_module', [
+    # should yield relative path to the parent directory
+    ('/foo/bar', ['/foo'], ['bar'], posixpath),
+    ('c:\\foo\\bar', ['c:\\foo'], ['bar'], ntpath),
+
+    # should not yield result if no path is a parent directory
+    ('/foo/bar', ['/baz'], [], posixpath),
+    ('c:\\foo\\bar', ['c:\\baz'], [], ntpath),
+
+    # should only yield relative paths for parent directories
+    ('/foo/bar', ['/foo', '/baz'], ['bar'], posixpath),
+    ('c:\\foo\\bar', ['c:\\foo', 'c:\\baz'], ['bar'], ntpath),
+
+    # should yield all results when multiple parents match
+    ('/foo/bar/baz', ['/foo', '/foo/bar'], ['bar/baz', 'baz'], posixpath),
+    ('c:\\foo\\bar\\baz', ['c:\\foo', 'c:\\foo\\bar'], ['bar\\baz', 'baz'], ntpath),
+
+    # should ignore case differences on windows
+    ('c:\\Foo\\bar', ['c:\\foo'], ['bar'], ntpath),
+
+    # should preserve original case
+    ('/Foo/Bar', ['/Foo'], ['Bar'], posixpath),
+    ('c:\\Foo\\Bar', ['c:\\foo'], ['Bar'], ntpath),
+])
+def test_relative_paths(value, paths, expected, path_module):
+    assert list(_relative_paths(value, paths, path_module)) == expected
+
+
+@pytest.mark.parametrize('value,paths,expected,path_module', [
+    # should yield relative path to the parent directory
+    ('/foo/bar', ['/foo'], 'bar', posixpath),
+    ('c:\\foo\\bar', ['c:\\foo'], 'bar', ntpath),
+    
+    # should return the original value if no path is a parent directory
+    ('/foo/bar', ['/baz'], '/foo/bar', posixpath),
+    ('c:\\foo\\bar', ['c:\\baz'], 'c:\\foo\\bar', ntpath),
+
+    # should yield shortest result when multiple parents match
+    ('/foo/bar/baz', ['/foo', '/foo/bar'], 'baz', posixpath),
+    ('c:\\foo\\bar\\baz', ['c:\\foo', 'c:\\foo\\bar'], 'baz', ntpath),
+])
+def test_shortest_relative_path(value, paths, expected, path_module):
+    assert _shortest_relative_path(value, paths, path_module) == expected