contrib/git-jump: add whitespace-checking mode

If you have whitespace errors in lines you've introduced, it
can be convenient to be able to jump directly to them for
fixing.  You can't quite use "git jump diff" for this,
because though it passes arbitrary options to "git diff", it
expects to see an actual unified diff in the output.

Whereas "git diff --check" actually produces lines that look
like compiler quickfix lines already, meaning we just need
to run it and feed the output directly to the editor.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/git-jump/README

@@ -29,7 +29,7 @@ Obviously this trivial case isn't that interesting; you could just open
 `foo.c` yourself. But when you have many changes scattered across a
 project, you can use the editor's support to "jump" from point to point.
 
-Git-jump can generate three types of interesting lists:
+Git-jump can generate four types of interesting lists:
 
   1. The beginning of any diff hunks.
 
@@ -37,6 +37,8 @@ Git-jump can generate three types of interesting lists:
 
   3. Any grep matches.
 
+  4. Any whitespace errors detected by `git diff --check`.
+
 
 Using git-jump
 --------------

contrib/git-jump/git-jump

@@ -12,6 +12,8 @@ diff: elements are diff hunks. Arguments are given to diff.
 merge: elements are merge conflicts. Arguments are ignored.
 
 grep: elements are grep hits. Arguments are given to grep.
+
+ws: elements are whitespace errors. Arguments are given to diff --check.
 EOF
 }
 
@@ -55,6 +57,10 @@ mode_grep() {
 	'
 }
 
+mode_ws() {
+	git diff --check "$@"
+}
+
 if test $# -lt 1; then
 	usage >&2
 	exit 1