You can use the C-x v ! (vc-edit-next-command) prefix
command to edit the shell command line that VC is about to run. This
is primarily intended to make it possible to add optional command-line
arguments to VCS commands without unnecessary complications of the VC
command set and its interfaces with the backend.
For example, Git can produce logs of more than one branch, but
C-x v b l (vc-print-branch-log) prompts for the name of
just one branch. To obtain a log of more than one branch, you can
type C-x v ! C-x v b l and then append the names of
additional branches to the end of the ‘git log’ command that VC
is about to run.