pg_rewind: Extend code detecting relation files to work with WAL files

pg_rewind: Extend code detecting relation files to work with WAL files

isRelDataFile() is renamed to getFileContentType(), extended so as it
becomes able to detect more file patterns than only relation files.  The
new file name pattern that can be detected is WAL files.

This refactoring has been suggested by Robert Haas.  This will be used
in a follow-up patch where we are looking at improving how WAL files are
processed by pg_rewind.  As of this change, WAL files are still handled
the same way as previously, always copied from the source to the target
server.

Extracted from a larger patch by the same authors.

Author: John Hsu <johnhyvr@gmail.com>
Author: Justin Kwan <justinpkwan@outlook.com>
Reviewed-by: Japin Li <japinli@hotmail.com>
Reviewed-by: Srinath Reddy Sadipiralla <srinath2133@gmail.com>
Discussion: https://postgr.es/m/181b4c6fa9c.b8b725681941212.7547232617810891479@viggy28.dev