Document git-diff whitespace flags -b and -w

Document git diff options -b / --ignore-space-change and
-w / --ignore-all-space, introduced by Johannes Schindelin
in commit 0d21efa, "Teach diff about -b and -w flags".

The description of options is taken from GNU diff man page and
GNU Diffutils info documentation.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/diff-options.txt

@@ -129,5 +129,21 @@
 -a::
 	Shorthand for "--text".
 
+--ignore-space-change::
+	Ignore changes in amount of white space.  This ignores white
+	space at line end, and consider all other sequences of one or
+	more white space characters to be equivalent.
+
+-b::
+	Shorthand for "--ignore-space-change".
+
+--ignore-all-space::
+	Ignore white space when comparing lines.  This ignores
+	difference even if one line has white space where the other
+	line has none.
+
+-w::
+	Shorthand for "--ignore-all-space".
+
 For more detailed explanation on these common options, see also
 link:diffcore.html[diffcore documentation].