[D7] Fix installation requirements

If you're getting "error" instead of "ok" then it means (as per README.txt) that the git command is not executable by your webserver, i.e. either the functions "exec" or "git" aren't available (e.g. git isn't installed or the functions are disabled in your php.ini).

Once these two functions are available the class will change to "ok".

This is because in the info file the text for the message is fixed with "The git binary is present and executable"
while the severity is changed in function of the existence or unexistence of the git and exec commands.
The severity determines then the icon to show, but the text is always the same.

The info file says:

function git_deploy_requirements($phase) {
  $requirements = array();
  if ($phase == 'runtime') {
    $requirements['git'] = array(
      'title' => t('git binary'),
      'value' => t('The git binary is present and executable'),
      'severity' => function_exists('exec') && exec('git') ? REQUIREMENT_OK : REQUIREMENT_ERROR,
    );
  }
  return $requirements;
}

We can test the requisites separately and set an understandable message according the problem. The severity ist set to ERROR if there is any error.

I have added a patch.

+1 the current message is confusing.

Ported fix from #3079539: Run exec() checking when module is installed as it would not work at all without it.

Uploaded wrong patch.