Make xxh64 the "main_name" for the current xxhash.

checksum.c

@@ -59,8 +59,8 @@ struct csum_struct {
 	const char *name, *main_name;
 } valid_checksums[] = {
 #ifdef SUPPORT_XXHASH
-	{ CSUM_XXH64, "xxhash", NULL },
 	{ CSUM_XXH64, "xxh64", NULL },
+	{ CSUM_XXH64, "xxhash", NULL },
 #endif
 	{ CSUM_MD5, "md5", NULL },
 	{ CSUM_MD4, "md4", NULL },
@@ -270,7 +270,7 @@ void negotiate_checksum(int f_in, int f_out, const char *csum_list, int fail_if_
 			if (!cs || !saw[cs->num] || best <= saw[cs->num])
 				continue;
 			xfersum_type = checksum_type = cs->num;
-			negotiated_csum_name = cs->name;
+			negotiated_csum_name = cs->main_name ? cs->main_name : cs->name;
 			best = saw[cs->num];
 			if (best == 1)
 				break;

rsync.yo

@@ -1376,7 +1376,7 @@ and (assuming bf(--checksum) is specified) the pre-transfer checksums. If two
 comma-separated names are supplied, the first name affects the transfer
 checksums, and the second name affects the pre-transfer checksums (bf(-c)).
 
-The algorithm choices are "auto", "xxhash", "MD5", "MD4", and "none".
+The algorithm choices are "auto", "xxh64" (aka "xxhash"), "MD5", "MD4", and "none".
 
 If "none" is specified for the first (or only) name, the bf(--whole-file) option
 is forced on and no checksum verification is performed on the transferred data.
@@ -1389,7 +1389,7 @@ negotation between the client and the server as follows:
 If both the client and the server are at least version 3.2.0, they will
 exchange a list of checksum names and choose the first one in the list that
 they have in common.
-This typically means that they will choose xxhash if they both support it
+This typically means that they will choose xxh64 if they both support it
 and fall back to MD5.
 If one side of the transfer is not new enough to support this checksum
 negotation, then a value is chosen based on the protocol version (which