Rename and refactor mch_FullName.

* Rename mch_FullName to mch_full_name to match the style guide.
* Add mch_full_dir_name, which saves the absolute path of a given
directory relative to cwd into a given buffer.
* Add function append_path, which glues together two given paths with a
slash.
* Adapt moonscript coding style to the tests.

src/fileio.c

@@ -4769,7 +4769,7 @@ void shorten_fnames(int force)
         && !path_with_url(buf->b_fname)
         && (force
             || buf->b_sfname == NULL
-            || mch_isFullName(buf->b_sfname))) {
+            || mch_is_full_name(buf->b_sfname))) {
       vim_free(buf->b_sfname);
       buf->b_sfname = NULL;
       p = shorten_fname(buf->b_ffname, dirname);