Git - testing whether merge is possible

[Rune Zedeler]

I am a bit surprised that I found no git-documentation on how to test 
whether it will be possible to merge two branches without actually 
recording the merge.

If I understand the docs correctly, this has to be done like this:

$ git merge --squash ...
$ git reset --mixed HEAD

... (test whether working tree is indeed working)

$ git reset --hard HEAD

Which I really find non-trivial. Especially I do not like that you have 
to update the index and reset it again at the start. I would prefer to 
be able to do something like this without touching anything but the 
working tree.
Have I misunderstood something?

-Rune