String value unsharing refactored into proper function.

All the Redis functions that need to modify the string value of a key in
a destructive way (APPEND, SETBIT, SETRANGE, ...) require to make the
object unshared (if refcount > 1) and encoded in raw format (if encoding
is not already REDIS_ENCODING_RAW).

This was cut & pasted many times in multiple places of the code. This
commit puts the small logic needed into a function called
dbUnshareStringValue().

src/object.c

@@ -660,7 +660,7 @@ unsigned long long estimateObjectIdleTime(robj *o) {
     }
 }
 
-/* This is a helper function for the DEBUG command. We need to lookup keys
+/* This is a helper function for the OBJECT command. We need to lookup keys
  * without any modification of LRU or other parameters. */
 robj *objectCommandLookup(redisClient *c, robj *key) {
     dictEntry *de;