CMake: extend add_version_info_from_vcs

CMake add_version_info_from_vcs function now requires 2nd SOURCE_DIR argument
specifying where to lookup repository information.

It also tries now to figure out repository URL and revision on Git mirror
parsing git-svn-id: footer from last commit (if present).

This will be used by Clang to show full build information when
LLVM_APPEND_VC_REV is enabled and LLVM/Clang are built from Git.