To show the output of recently run git commands, press $ (or, if that
isn’t available, use M-x magit-process-buffer). This shows 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, using M-x
magit-toggle-git-debug.