Update args for git-cppr.sh and git-ppr.sh

In `git-cppr.sh`, `--my_remote` had short option `-u`, and in
`git-ppr.sh`, `--my_repo` also had short option `-u`. In addition,
`git-ppr.sh` short option for `--pr_msg_dir` was `-m`. This update
switches to the more reasonable short option `-m` for `--my_remote`
and `--my_repo`, and sets the short option for `--pr_msg_dir` to the
more reasonable `-d`

This commit also renames `--our_remote` in `git-cppr.sh` to
`--our_repo`, since it's passed unmodified to the `--our_repo` option
in `git-ppr.sh` and can be either a remote or a github repository
reference.
Killed unneeded output in `pr_target_branch_args`

`git checkout` invocations in `pr_target_branch_args` were polluting
the content of the `ppr_args` variable