pgsql: Fix pg_rewind with in-place tablespaces when source is remote

Fix pg_rewind with in-place tablespaces when source is remote

libpq_source.c would consider any result returned by
pg_tablespace_location() as a symlink, resulting in run-time errors like
that:
pg_rewind: error: file "pg_tblspc/NN" is of different type in source and target

In-place tablespaces are directories located in pg_tblspc/, returned as
relative paths instead of absolute paths, so rely on that to make the
difference with a normal tablespace and an in-place one.  If the path is
relative, the tablespace is handled as a directory.  If the path is
absolute, consider it as a symlink.

In-place tablespaces are only intended for development purposes, so like
363e8f9 no backpatch is done.  A test is added in pg_rewind with an
in-place tablespace and some data in it.

Author: Rui Zhao, Michael Paquier
Discussion: https://postgr.es/m/2b79d2a8-b2d5-4bd7-a15b-31e485100980.xiyuan.zr@alibaba-inc.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/bf227926d22b5cffba4b6724df0eb239a7037cbd

Modified Files
--------------
src/bin/pg_rewind/libpq_source.c  | 11 ++++++++++-
src/bin/pg_rewind/t/001_basic.pl  | 20 ++++++++++++++++++++
src/bin/pg_rewind/t/RewindTest.pm |  1 +
3 files changed, 31 insertions(+), 1 deletion(-)