Merge branch 'jn/unknown-index-extensions' into pu

A bit too alarming warning given when unknown index extensions
exist is getting revamped.

* jn/unknown-index-extensions:
  index: offer advice for unknown index extensions
  index: do not warn about unrecognized extensions

Author: gitster

advice.c

@@ -28,6 +28,7 @@ int advice_add_embedded_repo = 1;
 int advice_ignored_hook = 1;
 int advice_waiting_for_editor = 1;
 int advice_graft_file_deprecated = 1;
+int advice_unknown_index_extension = 1;
 int advice_checkout_ambiguous_remote_branch_name = 1;
 int advice_nested_tag = 1;
 int advice_submodule_alternate_error_strategy_die = 1;
@@ -89,6 +90,7 @@ static struct {
 	{ "ignoredHook", &advice_ignored_hook },
 	{ "waitingForEditor", &advice_waiting_for_editor },
 	{ "graftFileDeprecated", &advice_graft_file_deprecated },
+	{ "unknownIndexExtension", &advice_unknown_index_extension },
 	{ "checkoutAmbiguousRemoteBranchName", &advice_checkout_ambiguous_remote_branch_name },
 	{ "nestedTag", &advice_nested_tag },
 	{ "submoduleAlternateErrorStrategyDie", &advice_submodule_alternate_error_strategy_die },

advice.h

@@ -28,6 +28,7 @@ extern int advice_add_embedded_repo;
 extern int advice_ignored_hook;
 extern int advice_waiting_for_editor;
 extern int advice_graft_file_deprecated;
+extern int advice_unknown_index_extension;
 extern int advice_checkout_ambiguous_remote_branch_name;
 extern int advice_nested_tag;
 extern int advice_submodule_alternate_error_strategy_die;

read-cache.c

@@ -1758,7 +1758,17 @@ static int read_index_extension(struct index_state *istate,
 		if (*ext < 'A' || 'Z' < *ext)
 			return error(_("index uses %.4s extension, which we do not understand"),
 				     ext);
-		fprintf_ln(stderr, _("ignoring %.4s extension"), ext);
+		if (advice_unknown_index_extension) {
+			warning(_("ignoring optional %.4s index extension"), ext);
+			advise(_("This is likely due to the file having been written by a newer\n"
+				 "version of Git than is reading it. You can upgrade Git to\n"
+				 "take advantage of performance improvements from the updated\n"
+				 "file format.\n"
+				 "\n"
+				 "Run \"%s\"\n"
+				 "to suppress this message."),
+			       "git config advice.unknownIndexExtension false");
+		}
 		break;
 	}
 	return 0;