Documentation: pruning recipe for destructive filter-branch

Add a section about how to shrink a repository's size after running
git-filter-branch to remove large blobs from history.

This comes up every week or so on IRC, and the commands required to
handle every case are not very newbie-friendly, so hopefully writing
them down somewhere leads to fewer questions.

It may seem contradictory to document fallbacks for older Gits in
newer docs, but we want to point people at this as a FAQ answer, and
they will frequently not have the newest version installed.

Thanks to Björn Steinbrink and Junio C Hamano for comments and
corrections.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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