diff: add -I<regex> that ignores matching changes

Add a new diff option that enables ignoring changes whose all lines
(changed, removed, and added) match a given regular expression.  This is
similar to the -I/--ignore-matching-lines option in standalone diff
utilities and can be used e.g. to ignore changes which only affect code
comments or to look for unrelated changes in commits containing a large
number of automatically applied modifications (e.g. a tree-wide string
replacement).  The difference between -G/-S and the new -I option is
that the latter filters output on a per-change basis.

Use the 'ignore' field of xdchange_t for marking a change as ignored or
not.  Since the same field is used by --ignore-blank-lines, identical
hunk emitting rules apply for --ignore-blank-lines and -I.  These two
options can also be used together in the same git invocation (they are
complementary to each other).

Rename xdl_mark_ignorable() to xdl_mark_ignorable_lines(), to indicate
that it is logically a "sibling" of xdl_mark_ignorable_regex() rather
than its "parent".

Signed-off-by: Michał Kępień <michal@isc.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4013-diff-various.sh

@@ -6,6 +6,7 @@
 test_description='Various diff formatting options'
 
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/diff-lib.sh
 
 test_expect_success setup '
 
@@ -309,6 +310,7 @@ log -SF master --max-count=2
 log -GF master
 log -GF -p master
 log -GF -p --pickaxe-all master
+log -IA -IB -I1 -I2 -p master
 log --decorate --all
 log --decorate=full --all
 
@@ -449,4 +451,43 @@ test_expect_success 'diff-tree --stdin with log formatting' '
 	test_cmp expect actual
 '
 
+test_expect_success 'diff -I<regex>: setup' '
+	git checkout master &&
+	test_seq 50 >file0 &&
+	git commit -m "Set up -I<regex> test file" file0 &&
+	test_seq 50 | sed -e "s/13/ten and three/" -e "/7\$/d" >file0 &&
+	echo >>file0
+'
+test_expect_success 'diff -I<regex>' '
+	git diff --ignore-blank-lines -I"ten.*e" -I"^[124-9]" >actual &&
+	cat >expect <<-\EOF &&
+	diff --git a/file0 b/file0
+	--- a/file0
+	+++ b/file0
+	@@ -34,7 +31,6 @@
+	 34
+	 35
+	 36
+	-37
+	 38
+	 39
+	 40
+	EOF
+	compare_diff_patch expect actual
+'
+
+test_expect_success 'diff -I<regex> --stat' '
+	git diff --stat --ignore-blank-lines -I"ten.*e" -I"^[124-9]" >actual &&
+	cat >expect <<-\EOF &&
+	 file0 | 1 -
+	 1 file changed, 1 deletion(-)
+	EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'diff -I<regex>: detect malformed regex' '
+	test_expect_code 129 git diff --ignore-matching-lines="^[124-9" 2>error &&
+	test_i18ngrep "invalid regex given to -I: " error
+'
+
 test_done