hgmerge: fix diff+patch detection; cleanups
Details:
- actually show help message when no tools are found
- whitespace cleanup (stupid emacs)
- quote more variables
- simplify merge/diff3 error handling
port:4:export
port:4:vaportight
port:4:import/export
FIXME: history is wrong here
port:1:1:-:eggs:import
port:1:2:+:eggs:vaportight
port:1:3:+:eggs:import/export
port:0:2:+:spam:export
port:0:1:+:spam:import
port:4:import/export