completions/git: fix arg completion for third-party git commands, again

Commit 50e595503e (completions/git: fix completions for third-party git
commands, 2025-03-03) wasn't quite right, as we can see in the linked
reproduction:

	$ fish_trace=1 complete -C 'git machete add --onto '
	----> complete -C git-machete\ add\n--onto\

The recursive completion invocation contains a spurious newline, which means
that "--onto" is the command name.  The newline is produced by "string escape
-- add --onto" inside a command substitution.

Fix this by interpreting newlines as list separators, and then joining
by spaces.

Fixes #11319

(cherry picked from commit 360cfdb7ae)

share/completions/git.fish

@@ -2571,9 +2571,9 @@ function __fish_git_complete_custom_command -a subcommand
     set -e cmd[1] # Drop "git".
     set -l subcommand_args
     if argparse -s (__fish_git_global_optspecs) -- $cmd
-        set subcommand_args $argv[2..] # Drop the subcommand.
+        set subcommand_args (string escape -- $argv[2..]) # Drop the subcommand.
     end
-    complete -C "git-$subcommand $(string escape -- $subcommand_args) "(commandline -ct)
+    complete -C "git-$subcommand $subcommand_args "(commandline -ct)
 end
 
 # source git-* commands' autocompletion file if exists

tests/checks/git.fish

@@ -51,6 +51,10 @@ complete -C'git frobnicate --onto '
 #CHECK: onto1
 #CHECK: onto2
 
+complete -C'git frobnicate graft --onto '
+#CHECK: onto1
+#CHECK: onto2
+
 complete -C'git ' | grep '^add'\t
 # (note: actual tab character in the check here)
 #CHECK: add	Add file contents to the staging area