Motivation for this change

Changes in 1.29

o 674163 : –   html-build.stamp rule broken for out-of-tree builds with absolute paths
o 795744 : Too much escaped string - " & lt;child > " in description of " GtkOverlay as GtkBuildable " section
o 796011 : Crash in ScanDirectory caused by overlooked use of renamed dir variable
o 796012 : Several places in rebase.py incorrectly use match.groups(1) instead of match.group(1), one causes a crash

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.