fast-export: fix anonymized tag using original length
Commit
7f4075949686 (fast-export: tighten anonymize_mem() interface to
handle only strings, 2020-06-23) changed the interface used in anonymizing
strings, but failed to update the size of annotated tag messages to match
the new anonymized string.
As a result, exporting tags having messages longer than 13 characters
would create output that couldn't be parsed by fast-import,
as the data length indicated was larger than the data output.
Reset the message size when anonymizing, and add a tag with a "long"
message to the test.
Signed-off-by: Tal Kelrich <hasturkun@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>