read-cache.c: reduce unnecessary cache entry name copying

read-cache.c: reduce unnecessary cache entry name copying

575fa8a3 (read-cache: read data in a hash-independent way,
2019-02-19) added a new code to copy from the on-disk data into the
name member of the in-core cache entry, which is already done
immediately after that in a way that takes prefix-compression into
account.

Remove this code, as it is not just unnecessary, but also can be
reading beyond the on-disk data, when we are copying very long
prefix string from the previous entry.

Signed-off-by: ZheNing Hu <adlternative@gmail.com>
[jc: rewrote the log message with Réne's findings]
Signed-off-by: Junio C Hamano <gitster@pobox.com>