subshell: ensure compatibility with fish 3.8

The upcoming fish 3.8 will add a feature flag to officially deprecate
"%self," see

fish-shell/fish-shell@8d71eef

Unfortunately this can cause mc+fish to break for users who configured
fish with "set -U fish_features all" which opts into any new feature
flag, thus disabling %self expansion.

Prevent this potential breakage by using the recommended "$fish_pid",
which was introduced in fish 3.0.0 (December 2018).

Signed-off-by: Yury V. Zaytsev <yury@shurup.com>

src/subshell/common.c

@@ -1176,7 +1176,7 @@ init_subshell_precmd (char *precmd, size_t buff_size)
                     "functions -e fish_right_prompt;"
                     "functions -c fish_prompt fish_prompt_mc; end;"
                     "function fish_prompt;"
-                    "echo \"$PWD\">&%d; fish_prompt_mc; kill -STOP %%self; end\n",
+                    "echo \"$PWD\">&%d; fish_prompt_mc; kill -STOP $fish_pid; end\n",
                     command_buffer_pipe[WRITE], command_buffer_pipe[WRITE], subshell_pipe[WRITE]);
         break;