convert/gnuarch: recode cat-log parts to utf-8 to be hg.description friendly

convert/gnuarch: add cat-log payload to commit description

bisect: improve hg bisect -c (relative paths, error handling) hg bisect -c failed with a relative path or when the executable wasn't found. Use util.find_exe()+os.spawnl() instead of os.spawnlp() and improve the handling of errors (killed process, exe not found). Thanks to Georg Brandl for reporting it.