How to undo a git merge squash?

If you run git merge --squash <other-branch> the working tree and index are updated with what the result of the merge would be, but it doesn't create the commit. All you need to do is to run:

git commit

However, if you change your mind before committing and just want to abort the merge, you can simply run:

git reset --merge

You don't need to use the reflog.

If you change your mind before committing, you have these options:

Abort the merge with modern git syntax:

git merge --abort

And with older syntax:

git reset --merge

And really old-school:

git reset --hard

But actually, it is worth noticing that git merge --abort is only equivalent to git reset --merge given that MERGE_HEAD is present. This can be read in the git help for merge command.

git merge --abort is equivalent to git reset --merge when MERGE_HEAD is present.

After a failed merge, when there is no MERGE_HEAD, the failed merge can be undone with git reset --merge but not necessarily with git merge --abort, so they are not only old and new syntax for the same thing. Personally I find git reset --merge much more useful in everyday work.