clone: fix --sparse option with URLs

clone: fix --sparse option with URLs

The --sparse option was added to the clone builtin in d89f09c (clone:
add --sparse mode, 2019-11-21) and was tested with a local path clone
in t1091-sparse-checkout-builtin.sh. However, due to a difference in
how local paths are handled versus URLs, this mechanism does not work
with URLs.

Modify the test to use a "file://" URL, which would output this error
before the code change:

  Cloning into 'clone'...
  fatal: cannot change to 'file://.../repo': No such file or directory
  error: failed to initialize sparse-checkout

These errors are due to using a "-C <path>" option to call 'git -C
<path> sparse-checkout init' but the URL is being given instead of
the target directory.

Update that target directory to evaluate this correctly. I have also
manually tested that https:// URLs are handled correctly as well.

Acked-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>