MD5 sum is calculated while the option is disabled

With jEdit trunk r13853, Java heap terribly grows when
a large file is opened. For example, opening 10 MB
plain text requires about 70 MB of heap, and heap grows
to over 100 MB.

I verified this on jProfiler and find the hotspot in
the new method Buffer.calculateHash(). It allocates
large memory, 20 MB for getText() and 10~30 MB for
getBytes(), to calculate the hash, which is never used
since I don't use the new option about MD5 check.

For comparison, jEdit 4.3pre15 requires about 25 MB to
open the same file.

I'm happy enough if the calculation doesn't happen
until the option is enabled. But the memory consumption
might be another bug.