Git: multi-pack-index builtin always shows progress

[wilbaker]

Splitting this out as a separate issue from #177

The multi-pack-index builtin shows progress during its output. There is no way to turn it off right now. This means that when we have an error, stderr is filled with progress indicators and the actual error message may be truncated in our logging.

Update git to allow --no-progress and use this argument in the background operations.

[derrickstolee]

Here are some hints:

You can take most of the changes from #172, but with the following replacements:

1. builtin/multi-pack-index.c instead of builtin/commit-graph.c.
2. Documentation/git-multi-pack-index.txt instead of Documentation/git-commit-graph.c
3. t/t5319-multi-pack-index.sh instead of t/t5318-commit-graph.sh

[wilbaker]

Fixed by #186