In a self-contained pack file delta references might point to base
objects stored later in the file.
In this case we need to replace placeholders for external refs with the
actual base object and update the children references.

Fixes: go-git#484

Co-authored-by: Markus Wolf <mail@markus-wolf.de>

Some applications that depend on go-git may not need
to build using CGO. The new test ensures no new
regressions going forwards.

Signed-off-by: Paulo Gomes <pjbgf@linux.com>

Fixes regression in which applications that depend on
go-git could no longer build with CGO_ENABLED=0.

Signed-off-by: Paulo Gomes <pjbgf@linux.com>