bitbake: git-make-shallow: add script to make a git repo shallow

This script will be used by the git fetcher to create shallow mirror tarballs.

    usage: git-make-shallow [-h] [--ref REF] [--shrink] REVISION [REVISION ...]

    Remove the history of the specified revisions, then optionally filter the
    available refs to those specified.

    positional arguments:
      REVISION           a git revision/commit

    optional arguments:
      -h, --help         show this help message and exit
      --ref REF, -r REF  remove all but the specified refs (cumulative)
      --shrink, -s       shrink the git repository by repacking and pruning

While git does provide the ability to clone at a specific depth, and fetch all
remote refs at a particular depth, the depth is across all branches/tags, and
doesn't provide the flexibility we need, hence this script.

Refs (branches+tags) can be filtered, as the process of history removal scales
up rapidly with the number of refs. Even the existing `git fetch --depth=` is
extremely slow on an upstream kernel repository with all the branches and tags
kept.

This uses the same underlying mechanism to implement the history removal which
git itself uses (.git/shallow), and the results, when configured similarly, are
in line with the results git itself produces with `fetch --depth`.

(Bitbake rev: 0254020f0e1911c0eaf99111b91828d2a74a4ee1)

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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