If the GIT_SUBDIR environment variable is set, do not checkout the full sources
of the git repository, but perform a sparse checkout of the directory containing
the package. In this case, check only the commit history in this subdirectory.
We now import the upstream tag in the debian repository, so
this explodes as it tries to walk all upstream commits. Use
--first-parent so that merges only get added via the merge
commit.
Let's rename the tool to tools/fetch-distro. It's useful to be able to fetch
the distro directly. But when that functionality is added, the old name is
confusing.
Now --update/-u must be specified to update the commits.
--reference-if-able is used to speed up the clone of debian.
It saves about 75% of the download.
