Add a fish_delta helper function

This helps figuring out which functions, completions and config you've overridden.

doc_src/cmds/fish_delta.rst

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+fish_delta - compare functions and completions to the default
+==============================================================
+
+Synopsis
+--------
+
+.. synopsis::
+
+    fish_delta name ...
+    fish_delta [-f | --no-functions] [-c | --no-completions] [-C | --no-config] [-d | --no-diff] [-n | --new] [-V | --vendor=]
+    fish_delta [-h | --help]
+
+
+Description
+-----------
+
+The ``fish_delta`` function tells you, at a glance, which of your functions and completions differ from the set that fish ships.
+
+It does this by going through the relevant variables (:envvar:`fish_function_path` for functions and :envvar:`fish_complete_path` for completions) and comparing the files against fish's default directories.
+
+If any names are given, it will only compare files by those names (plus a ".fish" extension).
+
+By default, it will also use ``diff`` to display the difference between the files. If ``diff`` is unavailable, it will skip it, but in that case it also cannot figure out if the files really differ.
+
+The exit status is 1 if there was a difference and 2 for other errors, otherwise 0.
+
+Options
+-------
+
+The following options are available:
+
+**-f** or **--no-functions**
+    Stops checking functions
+
+**-c** or **--no-completions**
+    Stops checking completions
+
+**-C** or **--no-config**
+    Stops checking configuration files like config.fish or snippets in the conf.d directories.
+
+**-d** or **--no-diff**
+    Removes the diff display (this happens automatically if ``diff`` can't be found)
+
+**-n** or **--new**
+    Also prints new files (i.e. those that can't be found in fish's default directories).
+
+**-Vvalue** or **--vendor=value**
+   Determines how the vendor directories are counted. Valid values are:
+
+   - "default" - counts vendor files as belonging to the defaults. Any changes in other directories will be counted as changes over them. This is the default.
+   - "user" - counts vendor files as belonging to the user files. Any changes in them will be counted as new or changed files.
+   - "ignore" - ignores vendor directories. Files of the same name will be counted as "new" if no file of the same name in fish's default directories exists.
+
+**-h** or **--help**
+    Prints ``fish_delta``'s help (this).
+
+Example
+-------
+
+Running just::
+
+  fish_delta
+
+will give you a list of all your changed functions and completions, including diffs (if you have the ``diff`` command).
+
+It might look like this::
+
+  > fish_delta
+  New: /home/alfa/.config/fish/functions/battery.fish
+  Changed: /home/alfa/.config/fish/test/completions/cargo.fish
+  --- /home/alfa/.config/fish/test/completions/cargo.fish 2022-09-02 12:57:55.579229959 +0200
+  +++ /usr/share/fish/completions/cargo.fish      2022-09-25 17:51:53.000000000 +0200
+  # the output of `diff` follows
+
+The options are there to select which parts of the output you want. With ``--no-completions`` you can compare just functions, and with ``--no-diff`` you can turn off the ``diff`` display.
+
+To only compare your ``fish_git_prompt``, you might use::
+
+  fish_delta --no-completions fish_git_prompt
+
+which will only compare files called "fish_git_prompt.fish".