index-pack: document and test the --promisor option

The --promisor option of 'git index-pack' was created in 88e2f9e
(introduce fetch-object: fetch one promisor object, 2017-12-05) but was
untested. It is currently unused within the Git codebase, but that will
change in an upcoming change to 'git bundle unbundle' when there is a
filter capability.

For now, add documentation about the option and add a test to ensure it
is working as expected.

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

0 files changed, 0 insertions, 0 deletions