Refactor basebackup.c's _tarWriteDir() function.

Refactor basebackup.c's _tarWriteDir() function.

Sometimes, we replace a symbolic link that we find in the data
directory with an actual directory within the tarfile that we
create. _tarWriteDir was responsible both for making this
substitution and also for writing the tar header for the
resulting directory into the tar file. Make it do only the first
of those things, and rename to convert_link_to_directory.

Substantially larger refactoring of this source file is planned,
but this little bit seemed to make sense to commit
independently.

Discussion: http://postgr.es/m/CA+Tgmobz6tuv5tr-WxURe5JA1vVcGz85k4kkvoWxcyHvDpEqFA@mail.gmail.com