[git] Get parent branch and diff since it (suggestion included)

I use AI to review merge requests. The assistants I use all support git natively. However, they don't know how to check all changes since the parent branch, because the "parent branch" is not a standard git concept.

So every time I just give the AI the parent branch name or the command to get it.

It would be nice if the git MCP offered this function.

Here is the command I use; I think it is a reasonable definition of a parent branch:

Find the first commit on the current branch that is on a branch other than the current branch or any remote with the same name.
Print the name of any such branch.

current_branch=$(git rev-parse --abbrev-ref HEAD) && \
for commit in $(git rev-list HEAD); do \
  other_branch=$(git branch -a --contains $commit | \
                /usr/bin/grep -Ev -m 1 "^[ *]*(remotes/.*/)?$current_branch$" | \
                sed -r 's#^[ *]*(remotes/)?##' | \
                xargs); \
  [[ -n "$other_branch" ]] && echo "$other_branch" && break; \
done

Start by locating the git MCP server and its existing tool definitions, then compare how current tools expose repository history. Use the supplied shell command as the behavioral reference; done means a tool can identify the parent branch and report the diff since it, following the stated branch-filtering rule.