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
%% no changes%% no changes%% uncompressed contents in repothis is a test%% uncompress our working dir copythis is a test%% uncompress our new working dir copythis is a test