Merge branch 'fc/at-head'

Instead of typing four capital letters "HEAD", you can say "@" now,
e.g. "git log @".

* fc/at-head:
  Add new @ shortcut for HEAD
  sha1-name: pass len argument to interpret_branch_name()

diff --cc Documentation/revisions.txt
Simple merge

diff --cc cache.h
Simple merge

diff --cc refs.c
Simple merge

diff --cc revision.c
Simple merge

diff --cc sha1_name.c
Simple merge