mru: use double-linked list from list.h

mru: use double-linked list from list.h

Simplify mru.[ch] and related code by reusing the double-linked list
implementation from list.h instead of a custom one.
This commit is an intermediate step. Our final goal is to get rid of
mru.[ch] at all and inline all logic.

Mentored-by: Christian Couder <redacted>
Mentored by: Jeff King <redacted>
Signed-off-by: Olga Telezhnaia <redacted>
Signed-off-by: Junio C Hamano <redacted>