[Improvement] Do complete checkout when updating local branch fails

[GeertvanHorrik]

Today I had this:

INFO [07/28/15 11:14:11:99] Updating branch 'release/1.2.0'
ERROR [07/28/15 11:14:12:11] An unexpected error occurred:
LibGit2Sharp.MergeConflictException: 8 conflicts prevent checkout
at LibGit2Sharp.Core.Ensure.HandleError(Int32 result)
at LibGit2Sharp.Core.Proxy.git_checkout_tree(RepositorySafeHandle repo, ObjectId treeId, GitCheckoutOpts& opts)
at LibGit2Sharp.Repository.CheckoutTree(Tree tree, IList`1 paths, IConvertableToGitCheckoutOpts opts)
at LibGit2Sharp.Repository.Checkout(Tree tree, CheckoutOptions checkoutOptions, String headTarget, String refLogHeadSpec, Signature signature)
at LibGit2Sharp.Repository.Checkout(Branch branch, CheckoutOptions options, Signature signature)
at LibGit2Sharp.Branch.Checkout()

When trying to get a version with remote repositories. There are a few things that are wrong here:

1. We probably need to "lock" the directory (what if 2 builds (release and check-in build) are running at the same time?)
2. We need to do a full clone if somehow this fails

[GeertvanHorrik]

Preferrably, I would like to fix this in GitTools.Core and start using that inside GitVersion.

[JakeGinnivan]

We need to do a full clone if somehow this fails

I think the locking should be enough, any other exceptions are bugs in branch normalisation and we need to fix rather than masking the problem.

[JakeGinnivan]

Created #599 to track the locking