Merge branch 'gs/retire-mru'

Retire mru API as it does not give enough abstraction over
underlying list API to be worth it.

* gs/retire-mru:
mru: Replace mru.[ch] with list.h implementation

diff --cc Makefile
Simple merge

diff --cc builtin/pack-objects.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc packfile.c
Simple merge

diff --cc sha1_file.c
Simple merge