Make diff header lines more explicit about what the diff is about

This gives a more readable output when producing diff for a task
with multiple items (with_items).

lib/ansible/runner/action_plugins/copy.py

@@ -126,6 +126,7 @@ class ActionModule(object):
                     dest_contents = base64.b64decode(dest_contents)
                 else:
                     raise Exception("unknown encoding, failed: %s" % dest_result.result)
+                diff['before_header'] = destination
                 diff['before'] = dest_contents
 
         src = open(source)
@@ -137,6 +138,7 @@ class ActionModule(object):
             diff['src_larger'] = utils.MAX_FILE_SIZE_FOR_DIFF
         else:
             src.seek(0)
+            diff['after_header'] = source
             diff['after'] = src.read()
 
         return diff

lib/ansible/runner/action_plugins/template.py

@@ -104,7 +104,7 @@ class ActionModule(object):
             module_args = "%s src=%s dest=%s" % (module_args, xfered, dest)
 
             if self.runner.check:
-                return ReturnData(conn=conn, comm_ok=True, result=dict(changed=True), diff=dict(before=dest_contents, after=resultant))
+                return ReturnData(conn=conn, comm_ok=True, result=dict(changed=True), diff=dict(before_header=dest, after_header=source, before=dest_contents, after=resultant))
             else:
                 res = self.runner._execute_module(conn, tmp, 'copy', module_args, inject=inject)
                 res.diff = dict(before=dest_contents, after=resultant)

lib/ansible/utils/__init__.py

@@ -639,7 +639,15 @@ def get_diff(diff):
             if 'src_larger' in diff:
                 ret.append("diff skipped: source file size is greater than %d\n" % diff['src_larger'])
             if 'before' in diff and 'after' in diff:
-                differ = difflib.unified_diff(diff['before'].splitlines(True), diff['after'].splitlines(True), 'before', 'after', '', '', 10)
+                if 'before_header' in diff:
+                    before_header = "before: %s" % diff['before_header']
+                else:
+                    before_header = 'before'
+                if 'after_header' in diff:
+                    after_header = "after: %s" % diff['after_header']
+                else:
+                    after_header = 'after'
+                differ = difflib.unified_diff(diff['before'].splitlines(True), diff['after'].splitlines(True), before_header, after_header, '', '', 10)
                 for line in list(differ):
                     ret.append(line)
             return "".join(ret)