Handle redirect HTTP responses, in particular if a Bugzilla server
is redirecting from http to https.

We try to detect "Bugzilla URL base is over here" when we ask for
show_bug.cgi and remember that for future requests to the same BugServer
to avoid too many redirections.

Switch from caching connection on the BugServer to a global connection
cache, and rewrite the BugServer cache so to deal with the possibility
of redirections.

Instead of resetting the branch and applying the patches there,
move to a detached HEAD, apply patches to the detached HEAD and
then make a single update to the branch ref.

The only real advantage of this is that 'git reflog show <branchname>'
shows a single meaningful commit.

 - Always do 'git bz attach --edit'
 - Apply patches as a single mailbox
 - Make -u/--add-url kinder on the reflog

When there is a single bug referenced in the commits being attached,
allow using that instead of specifying a bug reference manually.

If the server supports XML-RPC (3.0 or newer), then filing via
xmlrpc.cgi has advantages:

 - We get a string error message instead of an HTML page
 - We can not specify priority/severity and let the server
   default it; this is important since there aren't standard
   priorities.

So, we try to first file via xmlrpc.cgi's Bug.create method and
if we get a 404, fall back to the old method of a form post.

The configuration for a couple of fields is changed to use the human
readable field names from Bug.create rather than the database-field
names of post_bug.cgi.

 default-bug-severity => default-severity
 default-rep-platform => default-platform

  Allow omitting bug reference when obvious
  Default to -e/--edit when only one commit

Only use the commit body as the bug description when filing a new bug
if no other description is specified; if another description is
entered, use the commit body as the description for the attachment.

Add a -e/--edit option to 'git-bz-attach' to bring up the
description/comment in an editor. Existing patches are shown
as commented out Obsoletes: lines that can be uncommented to
to obsolete the old patches.

Allowing specifying bz-tracker.<tracker>.default-product and
bz-tracker.<tracker>.default-component. This is useful when done
in the per-repository config.

Add a "Per-repository configuration" section in the docs with examples.