Improve venv shell activation in fish shell

Instead of mimicking a user typing the command we use
fish's builtin `--init-command` option.

src/poetry/utils/shell.py

@@ -106,7 +106,12 @@ def activate(self, env: VirtualEnv) -> int | None:
  cmd = f"{self._get_source_command()} {shlex.quote(str(activate_path))}"
 
  with env.temp_environ():
- args = ["-e", cmd] if self._name == "nu" else ["-i"]
+ if self._name == "nu":
+ args = ["-e", cmd]
+ elif self._name == "fish":
+ args = ["-i", "--init-command", cmd]
+ else:
+ args = ["-i"]
 
  c = pexpect.spawn(
  self._path, args, dimensions=(terminal.lines, terminal.columns)
@@ -120,14 +125,11 @@ def activate(self, env: VirtualEnv) -> int | None:
  c.sendline(f"emulate bash -c '. {shlex.quote(str(activate_path))}'")
  elif self._name == "xonsh":
  c.sendline(f"vox activate {shlex.quote(str(env.path))}")
- elif self._name == "nu":
- # If this is nu, we don't want to send the activation command to the
+ elif self._name in ["nu", "fish"]:
+ # If this is nu or fish, we don't want to send the activation command to the
  # command line since we already ran it via the shell's invocation.
  pass
  else:
- if self._name in ["fish"]:
- # Under fish, "\r" should be sent explicitly
- cmd += "\r"
  c.sendline(cmd)
 
  def resize(sig: Any, data: Any) -> None: