[MERGE/RFC] Let _format_version_tuple accept alphas/betas/RCs with 0 sub-releases

[Matt Nordhoff]

Someone in #bzr can't run "bzr version" because his/her sys.version_info
looks like this:

(2, 5, 2, 'alpha', 0)

which is explicitly refused by _format_version_tuple:

>     This also checks that the version is reasonable: the sub-release must be
>     zero for final releases, and non-zero for alpha, beta and preview.

Since Python.org -- or at least some distro packager or something --
uses version numbers like that, I think it should be changed.

If this is rejected, bzrlib.trace.report_bug() and
bzrlib.version.show_version() (and any other users) should at least be
changed to handle this more gracefully.
-- 

Yay useless GPG signatures
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