Enable option to use local git cache of git repo if there are network issues when trying to connect to remote

[botwhytho]

When running code like below, if there is no network connection, the code will fail, even if there is a local git copy on the machine.

Metacello new
    baseline: 'MyProject';
    repository: 'github://myproject';
    load.

[botwhytho]

I've coded workarounds like below, but considering I'm introspecting with instVarNamed: we probably need to expose error codes from libgit a bit more officially

[Metacello new
  baseline: 'ABaseline';
  repository: 'github://aPath';
  load. ]
on: IceGenericError
do: [:anError|
  (anError instVarNamed: #wrappedException) object = LGitErrorTypeEnum giterr_net
    ifTrue: [
      Metacello new
      baseline: 'ABaseline';
      repository: 'gitlocal://aPath';
      load.			
]