To show the output of recently run git commands, press $ (or, if that
isn’t available, M-x magit-process-buffer). This will show a buffer
containing a section per git invocation; as always press TAB to expand
or collapse them.
By default, git’s output is only inserted into the process buffer if it
is run for side-effects. When the output is consumed in some way,
also inserting it into the process buffer would be too expensive. For
debugging purposes, it’s possible to do so anyway by setting
magit-git-debug to t.