List changed files in git post-merge hook

The right Git command to list the changed files is diff-tree

Also the ORIG_HEAD and HEAD shortcuts may be used:

git diff-tree -r --name-only --no-commit-id ORIG_HEAD HEAD

(see also: List all the files for a commit in Git)

[upd]

Perhaps it is better to use HEAD@{1} in place of ORIG_HEAD. I.e.:

git diff-tree -r --name-only --no-commit-id HEAD@{1} HEAD

In case of git pull --ff-only command, when many commits can be added, HEAD@{1} (inside post-merge hook) gives the last commit before this command, while ORIG_HEAD gives just HEAD^ commit (at least in Git 2.1.4).

I think your best bet at that point is going to be the reflogs. If you've just fast-forwarded, the first line of HEAD's reflog will look like this:

63e21fb HEAD@{0}: merge origin/master: Fast-forward

So you should be able to print just the first line (git reflog -n 1), check if it matches merge .*: Fast-forward$, and if so, do git diff HEAD@{1} HEAD. (You do want to look at the reflogs to verify that there was a fast-forward merge, probably, unless you can be confident from your script that it's the only possibility now.)