Get libgit2 sources from LHCb repository (@clemenci)
This is the CMT version of !2330 (merged), needed to be able to build on machines not connected to the external network.
See merge request !2426 (merged) (cherry picked from commit 8642c93f)
This is the CMT version of !2330 (merged), needed to be able to build on machines not connected to the external network.
See merge request !2426 (merged) (cherry picked from commit 8642c93f)