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>

xdiff/xdiff.h

@@ -32,6 +32,7 @@ extern "C" {
 #define XDF_IGNORE_WHITESPACE (1 << 2)
 #define XDF_IGNORE_WHITESPACE_CHANGE (1 << 3)
 #define XDF_IGNORE_WHITESPACE_AT_EOL (1 << 4)
+#define XDF_PATIENCE_DIFF (1 << 5)
 #define XDF_WHITESPACE_FLAGS (XDF_IGNORE_WHITESPACE | XDF_IGNORE_WHITESPACE_CHANGE | XDF_IGNORE_WHITESPACE_AT_EOL)
 
 #define XDL_PATCH_NORMAL '-'

xdiff/xdiffi.c

@@ -329,6 +329,9 @@ int xdl_do_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
 	xdalgoenv_t xenv;
 	diffdata_t dd1, dd2;
 
+	if (xpp->flags & XDF_PATIENCE_DIFF)
+		return xdl_do_patience_diff(mf1, mf2, xpp, xe);
+
 	if (xdl_prepare_env(mf1, mf2, xpp, xe) < 0) {
 
 		return -1;

xdiff/xdiffi.h

@@ -55,5 +55,7 @@ int xdl_build_script(xdfenv_t *xe, xdchange_t **xscr);
 void xdl_free_script(xdchange_t *xscr);
 int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
 		  xdemitconf_t const *xecfg);
+int xdl_do_patience_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
+		xdfenv_t *env);
 
 #endif /* #if !defined(XDIFFI_H) */