Review the result of git-merge before the actual merge

I call this the "code review workflow" and do it all the time.

git merge --no-commit --no-ff branchname

Without the --no-ff flag, if Git can do a fast-forward then it will do that. (As expected, as in the case of a fast forward, there's no merge commit to create.)

I have this alias setup in .gitconfig for convenience:

rev = merge --no-ff --no-commit

So that I can simply do:

git rev branchname

The idea is that all features are developed in separate branches, and each feature is reviewed and merged by somebody other than the author. As other answers pointed out you can abort the merge with:

git reset --merge

and ask the author to make more changes.

To view the log with only the merge commits I use this other alias:

revlog = log --first-parent

This way the log becomes a timeline of the large steps: feature by feature rather than commit by commit.

As Marian Theisen suggested, you can do this to do the merge without committing

git merge --no-commit <branchname>

You can back out of that merge with

git reset --hard

Also, remember that it is always easy to change your mind and return to a previous state in Git. You can do a full merge, including commit, inspect the complete result and if you change your mind you can

git reset --hard HEAD^

to throw away the merge and be back at the commit before the merge.

In fact, at any point during the merge resolution, you can do

git reset --merge

To abort the merge and throw away just the merge changes.

Why take the trouble? Just do the merge, test it, and if you don't like it, then git reset --hard HEAD^ to move back to pre-merge state. Doing some temporary or half-way merge just increases your work whether you decide you want to keep the merge or not.