whatchanged: remove when built with WITH_BREAKING_CHANGES

As we made "git whatchanged" require "--i-still-use-this" and asked
the users to report if they still want to use it, the logical next
step is to allow us build Git without "whatchanged" to prepare for
its eventual removal.

If we were to follow the pattern established in 8ccc75c2 (remote:
announce removal of "branches/" and "remotes/", 2025-01-22), we can
do this together with the documentation update to officially list
that the command will be removed in the BreakingChanges document,
but let's just keep the changes separate just in case we want to
proceed a bit slower.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -114,12 +114,14 @@ struct log_config {
 	char *fmt_pretty;
 	char *default_date_mode;
 
+#ifndef WITH_BREAKING_CHANGES
 	/*
 	 * Note: git_log_config() does not touch this member and that
 	 * is very deliberate.  This member is only to be used to
 	 * resurrect whatchanged that is deprecated.
 	 */
 	int i_still_use_this;
+#endif
 };
 
 static void log_config_init(struct log_config *cfg)
@@ -274,8 +276,10 @@ static void cmd_log_init_finish(int argc, const char **argv, const char *prefix,
 		OPT__QUIET(&quiet, N_("suppress diff output")),
 		OPT_BOOL(0, "source", &source, N_("show source")),
 		OPT_BOOL(0, "use-mailmap", &mailmap, N_("use mail map file")),
+#ifndef WITH_BREAKING_CHANGES
 		OPT_HIDDEN_BOOL(0, "i-still-use-this", &cfg->i_still_use_this,
 				"<use this deprecated command>"),
+#endif
 		OPT_ALIAS(0, "mailmap", "use-mailmap"),
 		OPT_CALLBACK_F(0, "clear-decorations", NULL, NULL,
 			       N_("clear all previously-defined decoration filters"),
@@ -642,6 +646,7 @@ static int git_log_config(const char *var, const char *value,
 	return git_diff_ui_config(var, value, ctx, cb);
 }
 
+#ifndef WITH_BREAKING_CHANGES
 int cmd_whatchanged(int argc,
 		    const char **argv,
 		    const char *prefix,
@@ -678,6 +683,7 @@ int cmd_whatchanged(int argc,
 	log_config_release(&cfg);
 	return ret;
 }
+#endif
 
 static void show_tagger(const char *buf, struct rev_info *rev)
 {