prefix.mtree overwritten on selfupdate

[markd at macports.org]

Someone has filed a bug that prefix.mtree gets overwritten during
selfupdate.

http://trac.macports.org/projects/macports/ticket/10917 

I notice that the prefix.mtree at the top says:   # Please see the file
src/etc/mtree/README before making changes to this file.

Where is this readme file?  When would this file be modified?  Is this a
legitimate bug?

Mark