Include empty directories in the default state of acceptance tests

Before each scenario of the acceptance tests is run the Nextcloud server
is reset to a default state. To do this the full directory of the
Nextcloud server is commited to a local Git repository and then reset to
that commit when needed.

Unfortunately, Git does not support including empty directories in a
commit. Due to this, when the default state was restored, it could
happen that the file cache listed an empty directory that did not exist
because it was not properly restored (for example,
"data/appdata_*/css/icons"), and that in turn could lead to an error
when the directory was used.

Currently the only way to force Git to include an empty directory is to
add a dummy file to the directory (so it will no longer be empty,
but that should not be a problem in the affected directories, even if
the dummy file is not included in the file cache); although Git FAQ
suggests using a ".gitignore" file a ".keep" file was used instead, as
it conveys better its purpose.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>