Merge first n commits of branch to master

I have a branch from the most recent commit to master. the branch has two commits on it. I'd like to fast-forward merge the first of those commits onto master.

I know I could do a cherry pick, but I don't really want to diverge the master and branch if I can help it.

Current state:

Branch:       +--->B--->C
             /
Master: --->A

Cherry-pick option:

Branch:       +--->B--->C
             /
Master: --->A--->B'

Ideal option:

Branch:            +--->C
                  /
Master: --->A--->B

Is this possible?

Note that the commits are all pushed to the remote already; but no-one else currently has access to this particular repo so a push -f would be OK.

I seem to remember there might be some way to merge onto master while a branch is checked out... if that is true, I could simply check out commit B, then merge in that manner?

Solution

More specifically, you might want:

git merge --ff-only <commit-specifier>

which tells Git to do a fast-forward not-really-a-merge of the current branch, whatever that is, to the specified commit. Such an operation:

tests whether HEAD is an ancestor of <commit-specifier>: if not, fail, if so, continue on to...
advance the current branch so that it points to <commit-specifier> and read that commit into the index and work-tree (git read-tree, the "two tree merge" case—note that this too can fail, if you have uncommitted changes, or untracked files that are in the target commit).

Now, finding a specific hash that's N commits "ahead of" the current commit in the direction of some target commit can be kind of tricky. Consider, for instance:

...--o--o   <-- master
         \
          \      C--D
           \    /    \
            A--B      G--H   <-- branch
                \    /
                 E--F

The tip commit of branch is eight commits ahead of the tip commit of master—or is it? I think everyone agrees that A is one commit ahead of master, and B is two ahead. But what about C and E, and D and F? C is three ahead, but so is E; and D and F are both four ahead. But G is not five ahead of master: it seems to be seven ahead of master. Likewise, H aka branch seems to be eight ahead of master.

So if I told you to move three commits ahead, would you pick commit C, or E? Would you give me an error, or a choice? Why?

If you constrain things to a linear chain, we can enumerate the commits "between" master and branch, excluding master and including branch, with:

git rev-list --reverse master..branch

and then the N-th commit in the list is the commit hash to give to git merge --ff-only. But if there are branch-and-merge chains "between" these two points, the whole question becomes tricky.