kvs: correct code logic about what is an append

Problem: In PR #2547, a fix was added into the KVS to deal with duplicate
appends to a KVS entry.  However, a corner case was left in the code.
When an append is done to a KVS entry that does not exist, the append
is treated like a normal insertion.  This corner case should also be
counted as an "append".  Otherwise, duplicate appends could happen at
the beginning of a KVS entry when it is created.

Solution: Set the append flag to true when a KVS entry does not exist
and the append is treated like an insert.

Fixes #6207

src/modules/kvs/kvstxn.c

@@ -495,6 +495,10 @@ static int kvstxn_append (kvstxn_t *kt, json_t *dirent,
         /* entry not found, treat like normal insertion */
         if (treeobj_insert_entry (dir, final_name, dirent) < 0)
             return -1;
+        /* N.B. although this is an "insert", we still treat this as
+         * an "append".  If we don't, the "append" could be issued
+         * twice, leading to duplicated data.  See issue #6207. */
+        (*append) = true;
     }
     else if (treeobj_is_valref (entry)) {
         char ref[BLOBREF_MAX_STRING_SIZE];