Possible fixes:
Delete the contents of this folder, and then restart Visual Studio.
%LOCALAPPDATA%\Microsoft\VisualStudio\12.0\ComponentModelCache
Switch the version control plugin to TFS (or another provider) and then return it to the "Microsoft Git Provider"
Open another Git repository, then reopen the offline repo.
Remove unnecessary version control plugins (e.g. Ahnk SVN when using only Git projects)
The refs / head / master file is corrupt. Re-clone repo
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