Use libgit2 for deleting local references

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Force parameter is never used

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