Replace filepattern with pathspec for consistency

pathspec is the most widely used term, and is the one defined in
gitglossary.txt. <filepattern> was used only in the synopsys for git-add
and git-commit, and in git-add.txt. Get rid of it.

This patch is obtained with by running:

  perl -pi -e 's/filepattern/pathspec/' `git grep -l filepattern`

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>