I agree with tarsius that Forge doesn't appear to support that operation, but I disagree with his philosophy slightly. If a feature that doesn't exist would be useful to you, you should create it! I so rarely label github issues that I've never thought to create a label from within Magit, but maybe I can help some.
The API endpoint you need is documented at ...
First: a variable can not have two different values at the same time.
(setq smerge-command-prefix "\C-cv")
(setq smerge-command-prefix "\C-c\C-v")
Second: to use this prefix the above variable has to be set before smerge-mode is loaded. So either ensure that (setq smerge-command-prefix "\C-cv") is eval'd before you use (or ...
See C-hig (emacs)VC With A Merging VCS
If you open vc-dir with C-xvd you can mark files with m -- or M to mark all files in a particular state -- and then C-xvv will act on all of those files.
For Git specifically, Magit provides a nicer UI than VC. If you use Git a lot, then definitely check it out -- it's well worth learning to use.