githelp: make several strings localizable

githelp: cleanly abort if the `svn` command is unknown Previously, the warning was lost above the sea of KeyError stacktrace output.

githelp: fail gracefully in a couple cases where arguments are missing I didn't bother adding tests because the other commands that already handled missing arguments don't test these edge cases. I didn't read over all of the code, rather I scanned for `args` not being checked before indexing.