git-notify: Make abbreviating Gitweb URLs optional

git-notify: Make abbreviating Gitweb URLs optional

The SHA1 object name part of Gitweb URLs is now only shortened if the
user requested this by specifying the new "-z" option (or by setting
"notify.shortURLs").

While at it, also shorten the additional URL which references a diff in
e-mail notifications which don't include that diff inline because its
size exceeds the maximum number of bytes specified via "-s".

Note that while the abbreviated SHA1 object names will be unique at push
time, this cannot be guaranteed for the future, so the shortened URLs
might break some day.