Avoid dangling-pointer usage in pg_basebackup progress reports.
Ill-considered refactoring in
23a1c6578 led to progress_filename
sometimes pointing to data that had gone out of scope. The most
bulletproof fix is to hang onto a copy of whatever's passed in.
Compared to the work spent elsewhere per file, that's not very
expensive, plus we can skip it except in verbose logging mode.
Per buildfarm.
Discussion: https://postgr.es/m/
20220212211316.GK31460@telsasoft.com