remove `return 1` from `echo_error`

Exiting `echo_error` with an error status didn’t propagate to call sites, so let the function return cleanly.
rename `switch_b` to `switch`
adjust `switch` to handle `switch to blah`
catch missing argument in `switch`
fix function call in `git_remote_for_git_flow`
add `return 1` after `echo_error` calls
quote string argument to `git_remote_exists`
add error handling to `branchstat`
initialize local variables a single time
remove colons after `then`
document `redirect_to_null`
quote `$@` in `redirect_to_null`
explicitly require `redirects.sh`
documentation changes for `git.sh`
introduce gitflow helpers; move `git_remote_for_git_flow` to `gitflow_remote`
introduce `autopr` to accompany `autopush`