scalar: add --[no-]src option

scalar: add --[no-]src option

Some users have strong aversions to Scalar's opinion that the repository
should be in a 'src' directory, even though this creates a clean slate
for placing build artifacts in adjacent directories.

The new --no-src option allows users to opt out of the default behavior.

While adding options, make sure the usage output by 'scalar clone -h'
reports the same as the SYNOPSIS line in Documentation/scalar.txt.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>