Minor change in revision changing message
Some work towards following build desc branch
A step further
Still need to sort remotes handling out for this case
Merge branch 'lifecycle-branching' of https://code.google.com/p/muddle into lifecycle-branching

Conflicts:
	muddled/version_control.py
	tests/test_lifecycle_git.py
Remove some more "invocation" uses

(how did they creep in?)
Tidying up - add cached builder.db.get_checkout_vcs

This should make it quicker to retrieve the VCS for a checkout
when we do it many times, plus it handles checking it is a checkout
label, etc.
Rewrite "muddle branch-tree" to make more sense

I believe this new version is more predictable, and also better
explained, and more robust. Also, the default version does the
checks we would like done, which is good.
Add some more help text

to the branch-tree command, and in doing so also add a check for
an explicitly specified revision in the build description.
Some more tinkering