diff_opt: track whether flags have been set explicitly

The diff_opt infrastructure sets flags based on defaults and command
line options.  It is impossible to tell whether a flag has been set
as a default or on explicit request.  Update the structure so that
this detection is possible:

 * Add an extra "opt->touched_flags" that keeps track of all the
   fields that have been touched by DIFF_OPT_SET and DIFF_OPT_CLR.

 * You may continue setting the default values to the flags, like
   commands in the "log" family do in cmd_log_init_defaults(), but
   after you finished setting the defaults, you clear the
   touched_flags field;

 * And then you let the usual callchain call diff_opt_parse(),
   allowing the opt->flags be set or unset, while keeping track of
   which bits the user touched;

 * There is an optional callback "opt->set_default" that is called
   at the very beginning to let you inspect touched_flags and update
   opt->flags appropriately, before the remainder of the diffcore
   machinery is set up, taking the opt->flags value into account.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 9 insertions, 1 deletions

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.