fix file name too long

I had problems with server crashing because the file name was too
long.

I like getting hints about hostname and path, but don't like getting
errors depending on length of the url.  This approach uses sha1 digest
of the original url to create a shorter version, while retaining the
previous approach of appending.  Helps edit documents like this:

    http://interactive.blockdiag.com/seqdiag