Interpret --skip arguments as relative paths

The --skip arguments are interpreted as paths relative to the
directories to check (the "file" arguments passed to codespell).

The benefit is that "codespell ." and "codespell $PWD" behave
the same, you just pass relative paths to --skip. No more adding
"./" to directory patterns.

codespell_lib/_codespell.py

@@ -875,7 +875,8 @@ def main(*args):
 
         if os.path.isdir(filename):
             for root, dirs, files in os.walk(filename):
-                if glob_match.match(root):  # skip (absolute) directories
+                rel_root = os.path.relpath(root, filename)
+                if glob_match.match(rel_root):  # skip directory (rel. path)
                     del dirs[:]
                     continue
                 if is_hidden(root, options.check_hidden):  # dir itself hidden
@@ -884,20 +885,21 @@ def main(*args):
                     # ignore hidden files in directories
                     if is_hidden(file_, options.check_hidden):
                         continue
-                    if glob_match.match(file_):  # skip files
+                    if glob_match.match(file_):  # skip file (base name)
                         continue
-                    fname = os.path.join(root, file_)
-                    if glob_match.match(fname):  # skip paths
+                    rel_path = os.path.join(rel_root, file_)
+                    if glob_match.match(rel_path):  # skip file (rel. path)
                         continue
+                    abs_path = os.path.join(root, file_)
                     bad_count += parse_file(
-                        fname, colors, summary, misspellings, exclude_lines,
+                        abs_path, colors, summary, misspellings, exclude_lines,
                         file_opener, word_regex, ignore_word_regex, uri_regex,
                         uri_ignore_words, context, options)
 
-                # skip (relative) directories
+                # skip directory (base name)
                 dirs[:] = [dir_ for dir_ in dirs if not glob_match.match(dir_)]
 
-        elif not glob_match.match(filename):  # skip files
+        elif not glob_match.match(filename):  # skip file
             bad_count += parse_file(
                 filename, colors, summary, misspellings, exclude_lines,
                 file_opener, word_regex, ignore_word_regex, uri_regex,