add a "basic" diff config callback

The diff porcelain uses git_diff_ui_config to set
porcelain-ish config options, like automatically turning on
color. The plumbing specifically avoids calling this
function, since it doesn't want things like automatic color
or rename detection.

However, some diff options should be set for both plumbing
and porcelain. For example, one can still turn on color in
git-diff-files using the --color command line option. This
means we want the color config from color.diff.* (so that
once color is on, we use the user's preferred scheme), but
_not_ the color.diff variable.

We split the diff config into "ui" and "basic", where
"basic" is suitable for use by plumbing (so _most_ things
affecting the output should still go into the "ui" part).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-diff-files.c

@@ -21,7 +21,7 @@ int cmd_diff_files(int argc, const char **argv, const char *prefix)
 
 	prefix = setup_git_directory_gently(&nongit);
 	init_revisions(&rev, prefix);
-	git_config(git_default_config); /* no "diff" UI options */
+	git_config(git_diff_basic_config); /* no "diff" UI options */
 	rev.abbrev = 0;
 
 	if (!setup_diff_no_index(&rev, argc, argv, nongit, prefix))

builtin-diff-index.c

@@ -17,7 +17,7 @@ int cmd_diff_index(int argc, const char **argv, const char *prefix)
 	int result;
 
 	init_revisions(&rev, prefix);
-	git_config(git_default_config); /* no "diff" UI options */
+	git_config(git_diff_basic_config); /* no "diff" UI options */
 	rev.abbrev = 0;
 
 	argc = setup_revisions(argc, argv, &rev, NULL);

builtin-diff-tree.c

@@ -68,7 +68,7 @@ int cmd_diff_tree(int argc, const char **argv, const char *prefix)
 	int read_stdin = 0;
 
 	init_revisions(opt, prefix);
-	git_config(git_default_config); /* no "diff" UI options */
+	git_config(git_diff_basic_config); /* no "diff" UI options */
 	nr_sha1 = 0;
 	opt->abbrev = 0;
 	opt->diff = 1;

diff.c

@@ -178,6 +178,12 @@ int git_diff_ui_config(const char *var, const char *value)
 				return parse_funcname_pattern(var, ep, value);
 		}
 	}
+
+	return git_diff_basic_config(var, value);
+}
+
+int git_diff_basic_config(const char *var, const char *value)
+{
 	if (!prefixcmp(var, "diff.color.") || !prefixcmp(var, "color.diff.")) {
 		int slot = parse_diff_color_slot(var, 11);
 		color_parse(value, var, diff_colors[slot]);

diff.h

@@ -172,6 +172,7 @@ extern void diff_unmerge(struct diff_options *,
 #define DIFF_SETUP_USE_CACHE		2
 #define DIFF_SETUP_USE_SIZE_CACHE	4
 
+extern int git_diff_basic_config(const char *var, const char *value);
 extern int git_diff_ui_config(const char *var, const char *value);
 extern void diff_setup(struct diff_options *);
 extern int diff_opt_parse(struct diff_options *, const char **, int);