multipack: Introduce a parser for multi-pack-index files
This change is the first in a series to add support for git's
multi-pack-index. This should speed up large repositories significantly.
Part of: #5399
Change git_repository initialization to use a path
The constructor to git_repository is now called
'git_repository_open(path)'
and takes a path to a git repository instead of an existing ODB object.
Unit tests have been updated accordingly and the two test repositories
have been merged into one.
Signed-off-by: Vicent Marti <tanoku@gmail.com>