Compute the SHA1 sum only for regular files

The current checksumming code in directory_record.rb's `content_modified?` method will attempt to open & compute the checksum of any directory entry that comes up as changed. On OS X at least, that will also try and compute the checksum of local sockets and FIFOs, which not only fails but also confuses any program on the other side of that dirent.

This change checks for the regular-fileness of a changed directory entry and simply doesn't count those as having had their content changed if they're not a regular file.

I am pretty sure that this PR as it is doesn't pass muster (there's no test for it), but would like to submit it for discussion anyway. What would be the ideal way to test this? Should I make it a platform-specific test for Darwin (where I can reproduce the issue)?