Emacs: why shell-command "git log" works, but "git shortlog" doesn't?
man git-shortlog
If no revisions are passed on the command line and either standard input is not a terminal or there is no current branch, git shortlog will output a summary of the log read from standard input, without reference to the current repository.
You must explicitly provide reference to work in your case,
Use, git shortlog HEAD
instead.