Git checkout and merge without touching working tree

A simple and safe way to do this—without a push or a forced update—is to fetch feature1 into master:

(feature1)$ git fetch . feature1:master
From .
   4a6000d..8675309  feature1   -> master

The trick is using . to get the local feature1 ref. This is safer than forcibly updating the master branch, since it ensures the update is a fast-forward. (See the <refspec> parameter in the git-fetch documentation for details.)

Now that feature1 and master are the same, switching between them will not touch any files:

(feature1)$ git checkout master
Switched to branch 'master'
(master)$

[Edit] This is only a partial solution / workaround. See the actual answer by @djpohly below.

Firstly, you can push from anywhere. Doesn't matter what you have checked out, or whether the commits you want to push are in master.

git push REMOTE_REPO feature1:master

see git help push

Hint: git push remoteRepo localRef:remoteRef

As for bringing master to where you are now without fiddling with your working copy... You can force it like so:

# (while still on feature1 branch)
git checkout -B master origin/master

But this does a hard reset on master. ie it doesn't check for fast-forward.