git-blame.el: Add (require 'format-spec)

git-blame.el: Add (require 'format-spec)

c5022f57 (git-blame.el: Change how blame information is shown,
2009-09-29) taught the "M-x git-blame" mode to format its output
in a more interesting way, making use of the format-spec function.

format-spec is included in Emacs 23 and is a useful function.
Older emacsen can get it from Gnus.  In all emacsen, we need
to 'require it before use to avoid warnings:

 git-blame.el:483:1:Warning: the function `format-spec' is not known to be
     defined.

Reported-by: Sergei Organov <redacted>
Reported-by: Kevin Ryde <redacted>
Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>