Merge branch 'sg/blame-in-bare-start-at-head'

Merge branch 'sg/blame-in-bare-start-at-head'

"git blame -- path" in a non-bare repository starts blaming from
the working tree, and the same command in a bare repository errors
out because there is no working tree by definition.  The command
has been taught to instead start blaming from the commit at HEAD,
which is more useful.

* sg/blame-in-bare-start-at-head:
  blame: default to HEAD in a bare repo when no start commit is given