Is there a git-merge --dry-run option?

As noted previously, pass in the --no-commit flag, but to avoid a fast-forward commit, also pass in --no-ff, like so:

$ git merge --no-commit --no-ff $BRANCH

To examine the staged changes:

$ git diff --cached

And you can undo the merge, even if it is a fast-forward merge:

$ git merge --abort

Undoing a merge with git is so easy you shouldn't even worry about the dry run:

$ git pull $REMOTE $BRANCH
# uh oh, that wasn't right
$ git reset --hard ORIG_HEAD
# all is right with the world

EDIT: As noted in the comments below, if you have changes in your working directory or staging area you'll probably want to stash them before doing the above (otherwise they will disappear following the git reset above)

Just diff your current branch against the remote branch, this will tell you what is going to change when you do a pull/merge.

#see diff between current master and remote branch
git diff master origin/master

My solution is to merge backwards.

Instead of merging your branch into the remote "target" branch, merge that branch into yours.

git checkout my-branch
git merge origin/target-branch

You will see if there are any conflicts and can plan on how to solve them.

After that you can either abort the merge via git merge --abort, or (if there weren't any conflicts and merge has happened) roll back to previous commit via git reset --hard HEAD~1

I want to see just the conflicts (can't see them with diff3 in GitHub, yet). Leveraging heavily from this answer above, I came up with this:

git merge --no-commit --no-ff @{upstream}
git grep -l '<<<<<<< HEAD' | xargs -I % sh -c "echo -e '\n\e[93m%\n---\e[0m' && cat %"
git merge --abort

For me, this is for checking my PRs. You can substitute @{upstream} with whatever branch.

I hope that's helpful to someone.

If you want to fast forward from B to A, then you must make sure that git log B..A shows you nothing, i.e. A has nothing that B doesn't have. But even if B..A has something, you might still be able to merge without conflicts, so the above shows two things: that there will be a fast-forward, and thus you won't get a conflict.

I know this is theoretically off-topic, but practically very on-topic for people landing here from a Google search.

When in doubt, you can always use the Github interface to create a pull-request and check if it indicates a clean merge is possible.