Fix pg_rewind when pg_xlog is a symlink.

Heikki Linnakangas <heikki.linnakangas@iki.fi>

Commit: 0e42397f42b370798461cbf3358185b520329f9f
Author: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: 2015-08-03T12:32:06Z
Releases: 9.6.0
Fix pg_rewind when pg_xlog is a symlink.

pg_xlog is often a symlink, typically to a different filesystem. Don't
get confused and comlain about by that, and just always pretend that it's a
normal directory, even if it's really a symlink.

Also add a test case for this.

Backpatch to 9.5.

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