completions/git: Cache subcommand v2

This is sort of slow because it's called hundreds of times.

We used to have a cache, introduced in ad9b4290e, but it was removed
in fee5a9125a because it had
false-positives.

So what we do, because the issue is that this is called hundreds of
times per-commandline, we cache it keyed on the commandline.

This speeds up `complete -C'git sta'` by a factor of 2.3x.

share/completions/git.fish

@@ -565,6 +565,19 @@ end
 
 function __fish_git_needs_command
     # Figure out if the current invocation already has a command.
+    #
+    # This is called hundreds of times and the argparse is kinda slow,
+    # so we cache it as long as the commandline doesn't change.
+    set -l cmdline "$(commandline -c)"
+    if set -q __fish_git_cmdline; and test "$cmdline" = "$__fish_git_cmdline"
+        if set -q __fish_git_cmd[1]
+            echo -- $__fish_git_cmd
+            return 1
+        end
+        return 0
+    end
+    set -g __fish_git_cmdline $cmdline
+
     set -l cmd (commandline -opc)
     set -e cmd[1]
     argparse -s (__fish_git_global_optspecs) -- $cmd 2>/dev/null
@@ -576,9 +589,11 @@ function __fish_git_needs_command
     set -q _flag_info_path; and return 1
     if set -q argv[1]
         # Also print the command, so this can be used to figure out what it is.
+        set -g __fish_git_cmd $argv[1]
         echo $argv[1]
         return 1
     end
+    set -g __fish_git_cmd
     return 0
 end
 

tests/checks/git.fish

@@ -139,3 +139,5 @@ end
 
 $fish -c 'complete -C "git -C ./.gi"'
 # CHECK: ./.git/	Directory
+
+$fish -c 'complete -C "git diff -c"'