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 branch name to hgpoller default name. Fixes #2469

this allows multiple hgpoller for different branches of the same repo
Dockerfile: Fixes unmet dependencies
svnUriCanonicalize should never fail, even on invalid uris
Merge dvinella/buildbot:fix-hgpollername (PR #1208)
Merge pull request #1210 from nikolaik/master

Dockerfile: Fixes unmet dependencies
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
make the default warning pattern case insensitive

* WarningCountingShellCommand is a generic class so it seems reasonable to
  catch as many warnings as possible
Merge sa2ajj/buildbot:default-warning-pattern (PR #1212)
update manual/introduction to follow one-sentence-per-line policy
update manual/installation