Replace 'unlink' with 'rm' in tests.

Several Unix-like systems (eg. OpenBSD) don't have 'unlink' utility. 'rm' does
exactly the same job, so using 'rm' instead of 'unlink' makes dulwich tests
more portable.

Signed-off-by: Jelmer Vernooij <jelmer@samba.org>
Update NEWS.
Support creating unannotated tags.
Add objectish argument to porcelain.tag.
Add time and timezone arguments for tag.
Support str timezones.
Add bin/dulwich support for reset command.
Sort command names in bin/dulwich.
Drop support for Python < 2.7.
No longer build for Python 2.6.