merge-recursive --patience

Teach the merge-recursive strategy a --patience option to use the
"patience diff" algorithm, which tends to improve results when
cherry-picking a patch that reorders functions at the same time as
refactoring them.

To support this, struct merge_options and ll_merge_options gain an
xdl_opts member, so programs can use arbitrary xdiff flags (think
"XDF_IGNORE_WHITESPACE") in a git-aware merge.

git merge and git rebase can be passed the -Xpatience option to
use this.

[jn: split from --ignore-space patch; with documentation]

Signed-off-by: Justin Frankel <justin@cockos.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge-recursive.c

@@ -2,6 +2,7 @@
 #include "commit.h"
 #include "tag.h"
 #include "merge-recursive.h"
+#include "xdiff-interface.h"
 
 static const char *better_branch_name(const char *branch)
 {