urls: Also decode slashes in message IDs

urls: Also decode slashes in message IDs

In commit 2653fdbb2, we started encoding slashes in message IDs
presented to users. This allowed us to support message IDs containing
slashes, which are allowed per the RFC and have been seen in the wild.
However, we continue to store the original unencoded message IDs in the
database and unfortunately we neglected to reverse the decoding in the
functions for downloading patch diffs and mboxes. Address this now.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Fixes: 2653fdbb2 ("urls: Encode slashes in message IDs")
Closes: #518
Cc: Siddhesh Poyarekar <siddhesh@gotplt.org>
Cc: DJ Delorie <dj@delorie.com>