Optimize fish_config theme choose

Just following basic shellscript optimization:

- Remove a useless use of cat (`status get-file` takes microseconds,
`status get-file | cat` is on the order of a millisecond - slower with
bigger $PATH)
- Pipe, don't run in a loop
- Filter early

This reduces the time taken from 12ms to 6ms on one of my systems, and
6.5ms to 4.5ms on another.

This is paid on every single shell startup, including
non-interactively, so it's worth it.

There's more to investigate, but this is a good first pass.

share/functions/__fish_theme_cat.fish

@@ -6,15 +6,16 @@ function __fish_theme_cat -a theme_name
         echo >&2 Searched (__fish_theme_dir) "and `status list-files themes`"
         return 1
     end
-    set -l theme_data (__fish_config_with_file $theme_path cat)
+    set -l theme_data (if string match -q '/*' -- $theme_path; cat $theme_path; else status get-file $theme_path; end)
     set -l allowed_lines \
         '\s*' \
         '\s*#.*' \
         '\[(dark|light|unknown)\]' \
         (__fish_theme_variable_filter)
     set allowed_lines "^($(string join -- '|' $allowed_lines))\$"
-    for line in $theme_data
-        string match -rq -- $allowed_lines $line
+    printf '%s\n' $theme_data | string match -rvq -- $allowed_lines
+    and for line in $theme_data
+        string match -rq -- $allowed_lines $theme_data
         or printf >&2 "error: unsupported line in theme '%s': %s\n" $theme_name $line
     end
     string join \n $theme_data

share/functions/fish_config.fish

@@ -287,7 +287,7 @@ function __fish_config_theme_choose
 
     set -l color_theme
     __fish_config_theme_canonicalize
-    set -l theme_data (type -q cat && __fish_theme_cat $theme_name)
+    set -l theme_data (__fish_theme_cat $theme_name)
     or return
     set -l color_themes dark light unknown
     set -l theme_is_color_theme_aware false
@@ -340,7 +340,7 @@ function __fish_config_theme_choose
         end
 
         set -l color_theme
-        string join \n -- $theme_data |
+        string match -re -- (__fish_theme_variable_filter)'|^\[.*\]$' $theme_data |
             while read -lat toks
                 if $theme_is_color_theme_aware
                     for ct in $color_themes
@@ -354,8 +354,8 @@ function __fish_config_theme_choose
                     end
                 end
                 set -l varname $toks[1]
-                string match -rq -- (__fish_theme_variable_filter) "$varname"
-                or continue
+                string match -q '[*' -- $varname
+                and continue
                 # If we're supposed to set universally, remove any shadowing globals
                 # so the change takes effect immediately (and there's no warning).
                 if test $scope = -U; and set -qg $varname