support/download: don't over-remove files from git archives

support/download: don't over-remove files from git archives

When we now manually create git archives, we removed all .git-related
files. However, we also exclude empty directories.

This means that a directory which only had a .gitignore file is excluded
from the archive.

Fixes:
    http://autobuild.buildroot.org/results/2aa/2aa8954311f009988880d27b6e48af91bc74c346/
    http://autobuild.buildroot.org/results/b45/b45cceea99b9860ccf1c925eeda498a823b30903/
    http://autobuild.buildroot.org/results/5ae/5ae336052fd32057d9631649279e142a81f5651f/
    http://autobuild.buildroot.org/results/5fc/5fc3abf4a1aea677f576e16c49253d00720a8bef/

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>