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In order to more easily compute delta bases among objects that appear at the exact same path, add a --path-walk option to 'git pack-objects'. This option will use the path-walk API instead of the object walk given by the revision machinery. Since objects will be provided in batches representing a common path, those objects can be tested for delta bases immediately instead of waiting for a sort of the full object list by name-hash. This has multiple benefits, including avoiding collisions by name-hash. The objects marked as UNINTERESTING are included in these batches, so we are guaranteeing some locality to find good delta bases. After the individual passes are done on a per-path basis, the default name-hash is used to find other opportunistic delta bases that did not match exactly by the full path name. RFC TODO: It is important to note that this option is inherently incompatible with using a bitmap index. This walk probably also does not work with other advanced features, such as delta islands. Getting ahead of myself, this option compares well with --full-name-hash when the packfile is large enough, but also performs at least as well as the default in all cases that I've seen. RFC TODO: this should probably be recording the batch locations to another list so they could be processed in a second phase using threads. RFC TODO: list some examples of how this outperforms previous pack-objects strategies. (This is coming in later commits that include performance test changes.) Signed-off-by: Derrick Stolee <stolee@gmail.com>