Get libgit2 sources from LHCb repository (@clemenci)
See merge request !2330 (merged) (cherry picked from commit fd4c2f23)
To build the Git CondDB access library (GitEntityResolver) in old branches we download the sources of libgit2
from the official repository.
Unfortunately one of the build machines (lblhcbpr13) does not have external connectivity, and cannot access the official libgit2
repository, so I copied the tar file to the LHCb repository created for LBCORE-1808, so that it can be downloaded from any build machine.