Merge branch 'jk/do-not-printf-NULL' into maint

"git config" had a codepath that tried to pass a NULL to
printf("%s"), which nobody seems to have noticed.

* jk/do-not-printf-NULL:
  git_config_set_multivar_in_file: handle "unset" errors
  git_config_set_multivar_in_file: all non-zero returns are errors
  config: lower-case first word of error strings

config.c

@@ -108,7 +108,7 @@ static int handle_path_include(const char *path, struct config_include_data *inc
 
 	expanded = expand_user_path(path);
 	if (!expanded)
-		return error("Could not expand include path '%s'", path);
+		return error("could not expand include path '%s'", path);
 	path = expanded;
 
 	/*
@@ -950,7 +950,7 @@ static int git_default_branch_config(const char *var, const char *value)
 		else if (!strcmp(value, "always"))
 			autorebase = AUTOREBASE_ALWAYS;
 		else
-			return error("Malformed value for %s", var);
+			return error("malformed value for %s", var);
 		return 0;
 	}
 
@@ -976,7 +976,7 @@ static int git_default_push_config(const char *var, const char *value)
 		else if (!strcmp(value, "current"))
 			push_default = PUSH_DEFAULT_CURRENT;
 		else {
-			error("Malformed value for %s: %s", var, value);
+			error("malformed value for %s: %s", var, value);
 			return error("Must be one of nothing, matching, simple, "
 				     "upstream or current.");
 		}
@@ -2217,9 +2217,13 @@ void git_config_set_multivar_in_file(const char *config_filename,
 				     const char *key, const char *value,
 				     const char *value_regex, int multi_replace)
 {
-	if (git_config_set_multivar_in_file_gently(config_filename, key, value,
-						   value_regex, multi_replace) < 0)
-		die(_("Could not set '%s' to '%s'"), key, value);
+	if (!git_config_set_multivar_in_file_gently(config_filename, key, value,
+						    value_regex, multi_replace))
+		return;
+	if (value)
+		die(_("could not set '%s' to '%s'"), key, value);
+	else
+		die(_("could not unset '%s'"), key);
 }
 
 int git_config_set_multivar_gently(const char *key, const char *value,
@@ -2400,7 +2404,7 @@ int git_config_rename_section(const char *old_name, const char *new_name)
 #undef config_error_nonbool
 int config_error_nonbool(const char *var)
 {
-	return error("Missing value for '%s'", var);
+	return error("missing value for '%s'", var);
 }
 
 int parse_config_key(const char *var,