Add .revisions() implementation to *Checkout classes.

Add .revisions() implementation to *Checkout classes.

This permits to count the number of check-ins between two revisions. This will
be used by the CQ to determine if a try job 'expired', e.g. it is so old that it
can't be used anymore. The .revisions() function specifically count the number
of commits, instead of just doing arithmetic on the svn revision numbers.

R=cmp@chromium.org
BUG=
TEST=Manually:
  import checkout
  s = checkout.SvnCheckout('/path/to/chrome/src', None, None, None, 'svn://svn.chromium.org/chrome/trunk/src')
  s.revisions(148323, 148330)
  s.revisions(148323, None)
  g = checkout.GitCheckout('.', None, 'master')
  g.revisions('HEAD^^^^^^^^', None)
  g.revisions('HEAD^^', 'HEAD')


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=148546

[cmp, 2012-07-25 20:03:01]

lgtm with nit, but wondering about lack of test for this

https://chromiumcodereview.appspot.com/10821011/diff/1/checkout.py
File checkout.py (right):

https://chromiumcodereview.appspot.com/10821011/diff/1/checkout.py#newcode195
checkout.py:195: return None
should this raise NotImplementedError, instead?

[M-A Ruel, 2012-07-25 20:25:03]

https://chromiumcodereview.appspot.com/10821011/diff/1/checkout.py
File checkout.py (right):

https://chromiumcodereview.appspot.com/10821011/diff/1/checkout.py#newcode195
checkout.py:195: return None
On 2012/07/25 20:03:01, cmp wrote:
> should this raise NotImplementedError, instead?

I prefer to keep None being returned when it's impossible to get the value,
since it's simpler for the caller.

Note that I uploaded another patchset that traps the CalledProcessError, I had
forgot about trapping them properly.

[cmp, 2012-07-25 20:28:42]

lgtm

[commit-bot: I haz the power, 2012-07-26 13:19:27]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/maruel@chromium.org/10821011/2002

[commit-bot: I haz the power, 2012-07-26 13:22:52]

Change committed as 148546