pg_basebackup fails with long tablespace paths

My Salesforce colleague Thomas Fanghaenel observed that the TAP tests
for pg_basebackup fail when run in a sufficiently deeply-nested directory
tree. The cause appears to be that we rely on standard "tar" format
to represent the symlink for a tablespace, and POSIX tar format has a
hard-wired restriction of 99 bytes in a symlink's expansion.

What do we want to do about this? I think a minimum expectation would be
for pg_basebackup to notice and complain when it's trying to create an
unworkably long symlink entry, but it would be far better if we found a
way to cope instead.

One thing we could possibly do without reinventing "tar" is to avoid using
absolute path names if a PGDATA-relative one would do.

regards, tom lane