perf(fish): migrate FORGIT_ vars without spawning subprocesses

The backwards-compatibility block that exports unexported FORGIT_ config
variables ran `set | awk | grep` plus a `set -x | grep` for every variable,
forking multiple external processes on every interactive fish startup.

Replace the pipeline with fish's built-in `set -n`/`set -xn` and
`string match`, which run in-process. Behavior is unchanged: unexported
FORGIT_ variables are still detected, warned about, and exported; nothing
is emitted when all variables are already exported.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

conf.d/forgit.plugin.fish

@@ -15,8 +15,8 @@ end
 # backwards compatibility:
 # export all user-defined FORGIT variables to make them available in git-forgit
 set unexported_vars 0
-set | awk -F ' ' '{ print $1 }' | grep FORGIT_ | while read var
-    if not set -x | grep -q "^$var\b"
+for var in (set -n | string match -er '^FORGIT_')
+    if not set -xn | string match -qr "^$var\$"
         if test $unexported_vars = 0
             forgit::warn "Config options have to be exported in future versions of forgit."
             forgit::warn "Please update your config accordingly:"