Merge branch 'jc/blame-reverse'

Merge branch 'jc/blame-reverse'

It is a common mistake to say "git blame --reverse OLD path",
expecting that the command line is dwimmed as if asking how lines
in path in an old revision OLD have survived up to the current
commit.

* jc/blame-reverse:
  blame: dwim "blame --reverse OLD" as "blame --reverse OLD.."
  blame: improve diagnosis for "--reverse NEW"