check for available git version

[xorpaul]

Testing g10k 0.4.5, which worked on CentOS 7, but I had to roll it back on CentOS 6.
I believe the problem is some incompatibility with the version of git available on CentOS 6.
This is the error that I get:

Executing git --git-dir /opt/puppetlabs/g10k/cache/ssh-__gitlab@gitlab.example.net_puppet-apps_perfui rev-parse --verify 'test^{object}' took 0.02260s
executeCommand(): git command failed: git --git-dir /opt/puppetlabs/g10k/cache/ssh-__gitlab@gitlab.example.net_puppet-apps_perfui rev-parse --verify 'test^{object}' exit status 128
Output: fatal: Needed a single revision

CentOS 6:

# git --version
git version 1.7.1

CentOS 7:

# git --version
git version 1.8.3.1

[xorpaul]

Added documentation for new g10k config setting:

To check for really existing objects, g10k uses master^{object} syntax, which is not supported in older Git versions, like on CentOS 6, see #91
g10k will skip this sanity check when the g10k config setting git_object_syntax_not_supported is set to true (defaults to false)
Example:

---
:cachedir: '/tmp/g10k'
git_object_syntax_not_supported: true

sources:
  example:
    remote: 'https://github.com/xorpaul/g10k-environment.git'
    basedir: '/tmp/example/'