It seems vim's trying to execute modelines in commit messages since 8c8cdf4405cb8bdb70dd9812a33bb52363a87dbc:
$ mkdir git1
$ git -C git1 init
$ git -C git1 commit --allow-empty 'initial'
$ git clone ./git1 ./git2 # make a clone so we get to see the "Unpushed to origin/main" section
$ git -C git2 commit --allow-empty -m 'vim: Update'
$ cd git2
$ vim -c Git
…
Error detected while processing modelines:
line 6:
E518: Unknown option: Update
It seems vim's trying to execute modelines in commit messages since 8c8cdf4405cb8bdb70dd9812a33bb52363a87dbc: