Note to self: It is bad to rely on Git's magic of finding out whether you can fast-forward when doing a merge. The more clever way is to explicitly state what you want to do with aliases:
nfm = merge --no-ff # no-ff-merge
ffm = merge --ff-only # ff-merge
I stumbled upon this today when I played around with subtree merges.
Strangely, the results with the strategy subtree were not the same
as with recursive's subtree=path option:
$ git merge -s recursive doc
$ git merge -Xsubtree=Documentation doc
Turns out that – in the special case I had created – Git could fast-forward the branch although it was a subtree merge, effectively ignoring any strategy option. Duh. I had already read, understood and tried all the tests related to merging before I had figured that out.