Releases should not be tested

See http://qa.drupal.org/pifr/test/132659 for an example.

When running tests on a fixed release (i.e. a tag rather than a branch), the testbot uses git clone -b <tag> <repository>, which fails because the fixed release is a tag, not a branch. The bot should be doing a git clone -n <repository> followed by a git checkout <tag> instead.

Here's an example (see below for the log): Issue is filed against Ubercart 6.x-2.4, patch is posted. Testbot tries git clone -b 6.x-2.4, which is the wrong command for checking out a tag. When this fails, the testbot falls back to checking out HEAD. However, HEAD is not used in Ubercart - it contains only a README.txt. So of course the patch doesn't apply and the test fails. (Even if HEAD were used for a project, it would likely be used for a Drupal 7 branch, not for Drupal 6.)

Here's an extract from the above log which shows what's happening:

[07:02:06] Command [git clone --reference /var/cache/git/reference -b '6.x' 'git://git.drupal.org/project/drupal.git' 'checkout' 2>&1] succeeded
  Duration: 4 seconds
  Directory: [/var/lib/drupaltestbot/sites/default/files]
  Completion status: [0]
  Output: [Cloning into checkout...].
[07:02:06] Main branch [git://git.drupal.org/project/drupal.git (6.x)] checkout complete.
[07:02:06] checkout_directory=sites/default/files/checkout; module_directory=sites/default/modules
[07:05:01] Command [git clone --reference /var/cache/git/reference -b '6.x-2.4' 'git://git.drupal.org/project/ubercart.git'  2>&1] succeeded
  Duration: 175 seconds
  Directory: [/var/lib/drupaltestbot/sites/default/files/checkout/sites/default/modules]
  Completion status: [0]
  Output: [Cloning into ubercart...
warning: Remote branch 6.x-2.4 not found in upstream origin, using HEAD instead].