Implement the patience diff algorithm

Implement the patience diff algorithm

The patience diff algorithm produces slightly more intuitive output
than the classic Myers algorithm, as it does not try to minimize the
number of +/- lines first, but tries to preserve the lines that are
unique.

To this end, it first determines lines that are unique in both files,
then the maximal sequence which preserves the order (relative to both
files) is extracted.

Starting from this initial set of common lines, the rest of the lines
is handled recursively, with Myers' algorithm as a fallback when
the patience algorithm fails (due to no common unique lines).

This patch includes memory leak fixes by Pierre Habouzit.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>