add 'git credential' plumbing command

add 'git credential' plumbing command

The credential API is in C, and not available to scripting languages.
Expose the functionalities of the API by wrapping them into a new
plumbing command "git credentials".

In other words, replace the internal "test-credential" by an official Git
command.

Most documentation writen by: Jeff King <redacted>
Signed-off-by: Pavel Volek <redacted>
Signed-off-by: Kim Thuat Nguyen <redacted>
Signed-off-by: Javier Roucher Iglesias <redacted>
Signed-off-by: Matthieu Moy <redacted>
Signed-off-by: Junio C Hamano <redacted>