git cherry-pick says "...38c74d is a merge but no -m option was given"

-m means the parent number.

From the git doc:

Usually you cannot cherry-pick a merge because you do not know which side of the merge should be considered the mainline. This option specifies the parent number (starting from 1) of the mainline and allows cherry-pick to replay the change relative to the specified parent.

For example, if your commit tree is like below:

- A - D - E - F -   master
   \     /
    B - C           branch one

then git cherry-pick E will produce the issue you faced.

git cherry-pick E -m 1 means using D-E, while git cherry-pick E -m 2 means using B-C-E.

The way a cherry-pick works is by taking the diff a changeset represents (the difference between the working tree at that point and the working tree of its parent), and applying it to your current branch.

So, if a commit has two or more parents, it also represents two or more diffs - which one should be applied?

You're trying to cherry pick fd9f578, which was a merge with two parents. So you need to tell the cherry-pick command which one against which the diff should be calculated, by using the -m option. For example, git cherry-pick -m 1 fd9f578 to use parent 1 as the base.

I can't say for sure for your particular situation, but using git merge instead of git cherry-pick is generally advisable. When you cherry-pick a merge commit, it collapses all the changes made in the parent you didn't specify to -m into that one commit. You lose all their history, and glom together all their diffs. Your call.