moveToExternal: Also check for utf8 encoding before trying to convert
While most rows in production use 'utf-8' to flag content being UTF-8,
we have lots of rows flagged with 'utf8':
mysql:research@s3-analytics-replica.eqiad.wmnet [dawiki]> select old_flags, count(*) from text group by old_flags limit 50;
+---------------------+----------+
| old_flags | count(*) |
+---------------------+----------+
| error | 2 |
| external,gzip | 49 |
| external,object | 36 |
| external,utf-8 | 1614469 |
| external,utf8 | 336780 |
| gzip,utf-8,external | 1094 |
| utf-8,gzip,external | 9458083 |
+---------------------+----------+
7 rows in set (26.038 sec)
This would confuse the script to try to reencode it again which possibly
could lead to all sorts of errors
Change-Id: I9b4a38538199c9954cfed51cdd2bba8b0f6cb953
(cherry picked from commit 4dd3850bebdd972001a007f1bcc5a127d3d4dc4f)