functions/fish_git_prompt: Let helper functions return useful status

If you use these to figure out if there _are_ staged files, or dirty
or whatever, you currently need to check the output, which relies on
the configured character.

Instead, we let them also return a useful status.
Notably, this is *not* simply the status of the git call.

__fish_git_prompt_X returns 0 if the repo is X.

This works for untracked, but the "diff" things return 1 if there is a
diff, so we invert the status for them.

See #5748.

[ci skip]

share/functions/fish_git_prompt.fish

@@ -485,36 +485,45 @@ end
 
 function __fish_git_prompt_staged --description "fish_git_prompt helper, tells whether or not the current branch has staged files"
     set -l sha $argv[1]
-
     set -l staged
+    set -l ret 0
 
     if test -n "$sha"
-        command git diff-index --cached --quiet HEAD -- 2>/dev/null
-        or set staged $___fish_git_prompt_char_stagedstate
+        # The "diff" functions all return > 0 if there _is_ a diff,
+        # but we want to return 0 if there are staged changes.
+        # So we invert the status.
+        not command git diff-index --cached --quiet HEAD -- 2>/dev/null
+        and set staged $___fish_git_prompt_char_stagedstate
+        set ret $status
     else
         set staged $___fish_git_prompt_char_invalidstate
+        set ret 2
     end
     echo $staged
+    return $ret
 end
 
 function __fish_git_prompt_untracked --description "fish_git_prompt helper, tells whether or not the current repository has untracked files"
-    set -l untracked
+    set -l ret 1
     if command git ls-files --others --exclude-standard --directory --no-empty-directory --error-unmatch -- :/ >/dev/null 2>&1
+        set ret $status
         set untracked $___fish_git_prompt_char_untrackedfiles
     end
     echo $untracked
+    return $ret
 end
 
 function __fish_git_prompt_dirty --description "fish_git_prompt helper, tells whether or not the current branch has tracked, modified files"
     set -l dirty
 
-    set -l os
-    command git diff --no-ext-diff --quiet --exit-code 2>/dev/null
-    set os $status
-    if test $os -ne 0
+    # Like staged, invert the status because we want 0 to mean there are dirty files.
+    not command git diff --no-ext-diff --quiet --exit-code 2>/dev/null
+    set -l os $status
+    if test $os -eq 0
         set dirty $___fish_git_prompt_char_dirtystate
     end
     echo $dirty
+    return $os
 end
 
 set -g ___fish_git_prompt_status_order stagedstate invalidstate dirtystate untrackedfiles