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