gitweb: Adding a `blame' interface.

gitweb: Adding a `blame' interface.

This patch adds an interface for `git-blame' to `gitweb.cgi'.
Links to it are placed in `git_blob'.

Internally the code uses `git-annotate' because `git-blame's output
differs for files that have been renamed in the past. However, I like
the term `blame' better.

[jc: blame can be told to produce the compatible format btw...]

Signed-off-by: Florian Forster <octo@verplant.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>