Disallow refs starting with a non-letter or digit

Change the regexp for validating refs to require at least one letter or digit
before allowing the other special chars in the set `[.-_]`. Names that start
with `.` are traditionally Unix hidden files; let's ignore them under the
assumption they're metadata for some other tool, and we don't want to
potentially conflict with the special `.` and `..` Unix directory entries.
Further, names starting with `-` are problematic for Unix cmdline option
processing; there's no good reason to support that. Finally, disallow `_` just
on general principle - it's simpler to say that ref identifiers must start with
a letter or digit.

We also ignore any existing files (that might be previously created refs) that
start with `.` in the `refs/` directory - there's a Red Hat tool for content
management that injects `.rsync` files, which is why this patch was first
written.

V1: Update to ban all refs starting with a non-letter/digit, and
    also add another call to `ostree_validate_rev` in the pull
    code.

Closes: https://github.com/ostreedev/ostree/issues/1285

Closes: #1286
Approved by: jlebon

src/libostree/ostree-repo-refs.c

@@ -560,6 +560,14 @@ enumerate_refs_recurse (OstreeRepo    *repo,
       if (dent == NULL)
         break;
 
+      /* https://github.com/ostreedev/ostree/issues/1285
+       * Ignore any files that don't appear to be valid fragments; e.g.
+       * Red Hat has a tool that drops .rsync_info files into each
+       * directory it syncs.
+       **/
+      if (!_ostree_validate_ref_fragment (dent->d_name, NULL))
+        continue;
+
       g_string_append (base_path, dent->d_name);
 
       if (dent->d_type == DT_DIR)