Add `git` to distribution packages

[caspiano]

Make distributions depend on git as shards requires git to function.

[straight-shoota]

FTR: This is about the DEB and RPM packages at https://build.opensuse.org/project/show/devel:languages:crystal

shards does not necessarily require git. Only if you want to install a shard from a git source (which is the most common, though). So I guess it could be an optional dependency of shards.
We currently don't have a separate shards package (shards is distributed in the crystal package), though.

Adding git as an (optional) dependency to crystal seems odd. Crystal itself really doesn't need git at all.