Merge branch 'jk/blame-contents-with-arbitrary-commit'

"git blame --contents=<file> <rev> -- <path>" used to be forbidden,
but now it finds the origins of lines starting at <file> contents
through the history that leads to <rev>.

* jk/blame-contents-with-arbitrary-commit:
  blame: allow --contents to work with non-HEAD commit

diff --cc blame.c
Simple merge