diff: fix bug with -I and overlapping hunks

Problem reported by Vincent Lefevre in <http://bugs.gnu.org/16864>.
* src/context.c (find_hunk): Threshold is CONTEXT only if
the second change is ignorable.
* tests/ignore-matching-lines: New test.
* tests/Makefile.am (TESTS): Add it.

src/context.c

@@ -402,9 +402,8 @@ find_hunk (struct change *start)
   lin top0, top1;
   lin thresh;
 
-  /* Threshold distance is 2 * CONTEXT + 1 between two non-ignorable
-     changes, but only CONTEXT if one is ignorable.  Watch out for
-     integer overflow, though.  */
+  /* Threshold distance is CONTEXT if the second change is ignorable,
+     2 * CONTEXT + 1 otherwise.  Watch out for integer overflow.  */
   lin non_ignorable_threshold =
     (LIN_MAX - 1) / 2 < context ? LIN_MAX : 2 * context + 1;
   lin ignorable_threshold = context;
@@ -416,7 +415,7 @@ find_hunk (struct change *start)
       top1 = start->line1 + start->inserted;
       prev = start;
       start = start->link;
-      thresh = (prev->ignore || (start && start->ignore)
+      thresh = (start && start->ignore
 		? ignorable_threshold
 		: non_ignorable_threshold);
       /* It is not supposed to matter which file we check in the end-test.

tests/Makefile.am

@@ -7,6 +7,7 @@ TESTS = \
   excess-slash \
   help-version	\
   function-line-vs-leading-space \
+  ignore-matching-lines \
   label-vs-func	\
   new-file \
   no-dereference \

tests/ignore-matching-lines

@@ -0,0 +1,47 @@
+#!/bin/sh
+# --ignore-matching-lines
+
+# Bug reported by Vincent Lefevre in <http://bugs.gnu.org/16864>.
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+
+fail=0
+
+cat <<'EOF' >a
+1a
+2
+3a
+4
+5
+6
+EOF
+
+cat <<'EOF' >b
+1b
+2
+3b
+4
+5
+6
+7
+EOF
+
+cat <<'EOF' >exp
+@@ -1,6 +1,7 @@
+-1a
++1b
+ 2
+-3a
++3b
+ 4
+ 5
+ 6
++7
+EOF
+
+diff -u --ignore-matching-lines 3 a b >out 2>err
+test $? = 1 || fail=1
+sed 1,2d out >outtail || framework_failure+
+compare exp outtail || fail=1
+
+Exit $fail