Whenever using `clone` command inside YADM_WORK sub-directory, we need
to checkout the correct repo contents.
Steps to reproduce:
```bash
mkdir $HOME/subdir
cd $HOME/subdir
yadm clone --bootstrap "<repo-url>"
yadm status
```
Instead of doing work to find the default branch just to be able to
set up the repository before doing a fetch, do a "git clone" and let
git handle it.
Use -c core.sharedrepository=0600 to get the same result as
--shared=0600 passed to init.
Use --separate-git-dir to get the git directory in $YADM_REPO. Use a
temporary dir as work tree and remove it right after the clone is
done.
When the clone is done, iterate over all missing files in $YADM_WORK
and perform a checkout. If local files exists that differ compared
with the cloned ones the local files are left intact and the user is
instructed to deal with the conflicts.
This makes it possible to run e.g. "yadm -Y foo introspect repo
2>/dev/null" and get an empty output instead of getting the error
message about foo not being fully qualified.
The new test system is written with py.test. These tests are more
comprehensive, run faster by an order of magnitude, and are far more
maintainable. The tests themselves conform to PEP8.
