diff: add --ignore-blank-lines option

The goal of the patch is to introduce the GNU diff
-B/--ignore-blank-lines as closely as possible. The short option is not
available because it's already used for "break-rewrites".

When this option is used, git-diff will not create hunks that simply
add or remove empty lines, but will still show empty lines
addition/suppression if they are close enough to "valuable" changes.

There are two differences between this option and GNU diff -B option:
- GNU diff doesn't have "--inter-hunk-context", so this must be handled
- The following sequence looks like a bug (context is displayed twice):

    $ seq 5 >file1
    $ cat <<EOF >file2
    change
    1
    2

    3
    4
    5
    change
    EOF
    $ diff -u -B file1 file2
    --- file1	2013-06-08 22:13:04.471517834 +0200
    +++ file2	2013-06-08 22:13:23.275517855 +0200
    @@ -1,5 +1,7 @@
    +change
     1
     2
    +
     3
     4
     5
    @@ -3,3 +5,4 @@
     3
     4
     5
    +change

So here is a more thorough description of the option:
- real changes are interesting
- blank lines that are close enough (less than context size) to
interesting changes are considered interesting (recursive definition)
- "context" lines are used around each hunk of interesting changes
- If two hunks are separated by less than "inter-hunk-context", they
will be merged into one.

The implementation does the "interesting changes selection" in a single
pass.

Signed-off-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

xdiff/xutils.c

@@ -143,6 +143,19 @@ long xdl_guess_lines(mmfile_t *mf, long sample) {
 	return nl + 1;
 }
 
+int xdl_blankline(const char *line, long size, long flags)
+{
+	long i;
+
+	if (!(flags & XDF_WHITESPACE_FLAGS))
+		return (size <= 1);
+
+	for (i = 0; i < size && XDL_ISSPACE(line[i]); i++)
+		;
+
+	return (i == size);
+}
+
 int xdl_recmatch(const char *l1, long s1, const char *l2, long s2, long flags)
 {
 	int i1, i2;