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 <osv@javad.com> Reported-by: Kevin Ryde <user42@zip.com.au> Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>