diff --git a/Documentation/technical/api-diff.txt b/Documentation/technical/api-diff.txt
index 2d2ebc04b7..8b001de0db 100644
--- a/Documentation/technical/api-diff.txt
+++ b/Documentation/technical/api-diff.txt
@@ -28,7 +28,8 @@ Calling sequence
 
 * Call `diff_setup_done()`; this inspects the options set up so far for
   internal consistency and make necessary tweaking to it (e.g. if
-  textual patch output was asked, recursive behaviour is turned on).
+  textual patch output was asked, recursive behaviour is turned on);
+  the callback set_default in diff_options can be used to tweak this more.
 
 * As you find different pairs of files, call `diff_change()` to feed
   modified files, `diff_addremove()` to feed created or deleted files,
@@ -115,6 +116,13 @@ Notable members are:
 	operation, but some do not have anything to do with the diffcore
 	library.
 
+`touched_flags`::
+	Records whether a flag has been changed due to user request
+	(rather than just set/unset by default).
+
+`set_default`::
+	Callback which allows tweaking the options in diff_setup_done().
+
 BINARY, TEXT;;
 	Affects the way how a file that is seemingly binary is treated.
 
diff --git a/builtin/log.c b/builtin/log.c
index ad46f72950..3abbc668a7 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -91,6 +91,7 @@ static void cmd_log_init_defaults(struct rev_info *rev)
 
 	if (default_date_mode)
 		rev->date_mode = parse_date_format(default_date_mode);
+	rev->diffopt.touched_flags = 0;
 }
 
 static void cmd_log_init_finish(int argc, const char **argv, const char *prefix,
diff --git a/diff.c b/diff.c
index f0b3e7cfe3..7c248726ad 100644
--- a/diff.c
+++ b/diff.c
@@ -3213,6 +3213,9 @@ void diff_setup_done(struct diff_options *options)
 {
 	int count = 0;
 
+	if (options->set_default)
+		options->set_default(options);
+
 	if (options->output_format & DIFF_FORMAT_NAME)
 		count++;
 	if (options->output_format & DIFF_FORMAT_NAME_STATUS)
diff --git a/diff.h b/diff.h
index 78b4091dd5..e995ae12fe 100644
--- a/diff.h
+++ b/diff.h
@@ -87,8 +87,9 @@ typedef struct strbuf *(*diff_prefix_fn_t)(struct diff_options *opt, void *data)
 #define DIFF_OPT_PICKAXE_IGNORE_CASE (1 << 30)
 
 #define DIFF_OPT_TST(opts, flag)    ((opts)->flags & DIFF_OPT_##flag)
-#define DIFF_OPT_SET(opts, flag)    ((opts)->flags |= DIFF_OPT_##flag)
-#define DIFF_OPT_CLR(opts, flag)    ((opts)->flags &= ~DIFF_OPT_##flag)
+#define DIFF_OPT_TOUCHED(opts, flag)    ((opts)->touched_flags & DIFF_OPT_##flag)
+#define DIFF_OPT_SET(opts, flag)    (((opts)->flags |= DIFF_OPT_##flag),((opts)->touched_flags |= DIFF_OPT_##flag))
+#define DIFF_OPT_CLR(opts, flag)    (((opts)->flags &= ~DIFF_OPT_##flag),((opts)->touched_flags |= DIFF_OPT_##flag))
 #define DIFF_XDL_TST(opts, flag)    ((opts)->xdl_opts & XDF_##flag)
 #define DIFF_XDL_SET(opts, flag)    ((opts)->xdl_opts |= XDF_##flag)
 #define DIFF_XDL_CLR(opts, flag)    ((opts)->xdl_opts &= ~XDF_##flag)
@@ -109,6 +110,7 @@ struct diff_options {
 	const char *single_follow;
 	const char *a_prefix, *b_prefix;
 	unsigned flags;
+	unsigned touched_flags;
 	int use_color;
 	int context;
 	int interhunkcontext;
@@ -145,6 +147,8 @@ struct diff_options {
 	/* to support internal diff recursion by --follow hack*/
 	int found_follow;
 
+	void (*set_default)(struct diff_options *);
+
 	FILE *file;
 	int close_file;