Use libgit2 for GetOneFile

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Use libgit2 for cat command

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Improve error handling of GetOneFile

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Use libgit2 for IsBranchTagNameUnique

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Use libgit2 for BranchTagExists

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Use libgit2 to get branch list

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Update libgit2

libgit2 team added basic support for core.safecrlf.

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Make GetHash callable with git_repository object

Signed-off-by: Sven Strickroth <email@cs-ware.de>