clone: ignore --depth when cloning locally (implicitly --local)
When cloning locally, we default to --local, as it makes the whole
operation fast and efficient.
As the most common intent of cloning with a --depth parameter is to
save space, and --local saves more space than --depth ever can,
warn the user and ignore the --depth parameter when cloning locally.
Should --depth be desired, the user can always force proper cloning
by passing the --no-hardlinks parameter, or by using a file:// URL.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>