add support for git tags in bit_buildbot.py

Tags are treated as branches, but only one change is sent to buildbot
Refactor process_changes() in git_buildbot.py

Split out the loop body, and separate branch and tag processing
Split process_changes() in git_buildbot.py

Separate harvesting changes from sending changes,
which go into send_changes()
Add a release note for git_buildbot.py tag support
re-format change source docs according to one-line-per-sentence policy
remove some extra spaces
fix documentation about change sources related to Mercurial
add a reference to the BitBucket change hook for Mercurial
Merge d4ddi0/buildbot:git_buildbot_tags (PR #1207)
Merge sa2ajj/buildbot:hg-docs-fix (PR #1211)
pep8, pyflakes