Don't call fundamental-mode if we already are in fundamental-mode

[plexus]

If the " *nrepl-decoding*" buffer already exists then it is likely already in
the right mode. Not re-initializing the mode means we don't re-trigger hooks,
which in this case in particular prevents an unfortunate interaction with
evil-mode, where the cursor color gets updated even though it shouldn't.

Fixes #3068

Replace this placeholder text with a summary of the changes in your PR.
The more detailed you are, the better.

---

Before submitting the PR make sure the following things have been done (and denote this
by checking the relevant checkboxes):

• The commits are consistent with our contribution guidelines
• You've added tests (if possible) to cover your change(s)
• All tests are passing (eldev test)
• All code passes the linter (eldev lint) which is based on elisp-lint and includes
  • byte-compilation, checkdoc, check-declare, packaging metadata, indentation, and trailing whitespace checks.
• You've updated the changelog (if adding/changing user-visible functionality)
• You've updated the user manual (if adding/changing user-visible functionality)

Thanks!

If you're just starting out to hack on CIDER you might find this section of its
manual extremely useful.

[vemv]

There's another call at

cider/nrepl-client.el

Line 1359 in 69606fa

(fundamental-mode)

- maybe we can extract a helper and apply it in both places?

[plexus]

• maybe we can extract a helper and apply it in both places?

Done!

[vemv]

☕️☕️

[plexus]

Sorry @vemv only 🍵 for me 🙏