Merge branch 'jk/tag-contains' into pu

* jk/tag-contains:
  Why is "git tag --contains" so slow?
  default core.clockskew variable to one day
  limit "contains" traversals based on commit timestamp
  tag: speed up --contains calculation

diff --cc .gitignore
Simple merge

diff --cc Makefile
Simple merge

diff --cc builtin.h
Simple merge

diff --cc builtin/tag.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc commit.c
Simple merge

diff --cc commit.h
Simple merge

diff --cc config.c
Simple merge

diff --cc git.c
Simple merge