Add support for mergetags.

Bug: https://bugs.launchpad.net/dulwich/+bug/963525
Simplify parsing for multi-line fields.
Fix PEP8 compatibility of setup.py.
web: Some formatting fixes.
Fix URL handling so that git+ssh:// is treated like git's ssh:// * fixed unit test as well

Signed-off-by: Jelmer Vernooij <jelmer@samba.org>
test_client: Put separate tests into their own methods.