diff: make the indent heuristic part of diff's basic configuration

This heuristic was originally introduced as an experimental feature,
and therefore part of the UI configuration.

But the user often sees diffs generated by plumbing commands like
diff-tree.  Moving the indent heuristic into diff's basic configuration
prepares the way for diff plumbing commands to respect the setting.

The heuristic itself merely makes the diffs more aesthetically
pleasing, without changing their correctness.  Scripts that rely on
the diff plumbing commands should not care whether or not the heuristic
is employed.

Signed-off-by: Marc Branchaud <marcnarc@xiplink.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -290,9 +290,6 @@ int git_diff_ui_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
-	if (git_diff_heuristic_config(var, value, cb) < 0)
-		return -1;
-
 	if (!strcmp(var, "diff.wserrorhighlight")) {
 		int val = parse_ws_error_highlight(value);
 		if (val < 0)
@@ -351,6 +348,9 @@ int git_diff_basic_config(const char *var, const char *value, void *cb)
 	if (starts_with(var, "submodule."))
 		return parse_submodule_config_option(var, value);
 
+	if (git_diff_heuristic_config(var, value, cb) < 0)
+		return -1;
+
 	return git_default_config(var, value, cb);
 }