abbr: Ensure we don't split on "=" if the given separator is " "

This fails on e.g. an abbr that uses `env a=b`, like the included test demonstrates.

Unfortunately it decreases the speed again (2s vs 2.2s vs 4s original),
but correctness is more important.

share/functions/abbr.fish

@@ -157,18 +157,20 @@ function __fish_abbr_parse_entry -S -a __input __key __value
 	if test -z "$__value"
 		set __value __
 	end
-	switch $__input
-		case "*=*"
-			# No need for bounds-checking because we already matched before
-			set -l KV (string split "=" -m 1 -- $__input)
-			set $__key $KV[1]
-			set $__value $KV[2]
-		case "* *"
-			set -l KV (string split " " -m 1 -- $__input)
-			set $__key $KV[1]
-			set $__value $KV[2]
-		case "*"
-			set $__key $__input
+	# A "=" _before_ any space - we only read the first possible separator
+	# because the key can contain neither spaces nor "="
+	if string match -qr '^[^ ]+=' -- $__input
+		# No need for bounds-checking because we already matched before
+		set -l KV (string split "=" -m 1 -- $__input)
+		set $__key $KV[1]
+		set $__value $KV[2]
+	else if string match -qr '^[^ ]+ .*' -- $__input
+		set -l KV (string split " " -m 1 -- $__input)
+		set $__key $KV[1]
+		set $__value $KV[2]
+	else
+		# This is needed for `erase` et al, where we want to allow passing a value
+		set $__key $__input
 	end
 	return 0
 end

tests/abbr.in

@@ -31,3 +31,7 @@ abbr -e '~__abbr2'
 abbr -- '--__abbr3' 'xyz'
 abbr | grep __abbr3
 abbr -e '--__abbr3'
+
+# Ensure we are not recognizing later "=" as separators
+abbr d2 env a=b banana
+abbr -l | string match -q d2; or echo "= test failed"