pgsql: Fix pg_rewind when pg_xlog is a symlink.

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.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/0e42397f42b370798461cbf3358185b520329f9f

Modified Files
--------------
src/bin/pg_rewind/RewindTest.pm | 14 +++--
src/bin/pg_rewind/filemap.c | 16 +++++-
src/bin/pg_rewind/t/001_basic.pl | 1 +
src/bin/pg_rewind/t/002_databases.pl | 1 +
src/bin/pg_rewind/t/003_extrafiles.pl | 1 +
src/bin/pg_rewind/t/004_pg_xlog_symlink.pl | 79 ++++++++++++++++++++++++++++
6 files changed, 107 insertions(+), 5 deletions(-)