Integrate support to detect changed recipes from git

[wolfv]

I wonder if this would be useful but some people are setting up repositories with a number of recipes in sub-folders. If we had some intelligent support to find which recipes changed, we could rebuild those individually.

This also has some overlap with "building-all-recipes" but we could save some time by not collecting some recipes / outputs and only rebuild what has changed.

Although this becomes more tricky if the variant config file changes (then we may have to rebuild more packages).