buildhistory-diff: improve bad command-line argument handling
* Check for existence of specified buildhistory directory and show a
proper error message if it doesn't
* Show an error message instead of a traceback with a mangled revision
if one of the specified git revisions is invalid
* Show usage information if --help is specified
* Write error messages to stderr
Fixes [YOCTO #4313].
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
