code review fixes

* clean up error handling: show Exception info
* white space fixes
* clean up output when creating CL
* simplify hg change command; add hg file
* fix stale cookie bug (thanks iant)
* in LoadAllCL, load each CL in a different thread,
  to parallelize the slow web fetches
* throw away support for Mercurial before version 1.3
* add @CL-number file pattern for commands like diff
* make hg sync show files being sync'ed

R=r
http://go/go-review/1016016