fish_add_path: Automatically enable verbose mode for interactive use

fish_add_path can be used either interactively, in the commandline,
or in config.fish. That's its greatest strength, it's a very
DWIM-style command.

One of the compromises that entails, however, is that it can't really
be very loud about what it does. If it skips a path, it can't write a
warning because it might be used in config.fish.

But it *can* if it's used interactively. So we try to detect that case
and enable verbose mode automatically.

That means if you do

```fish
fish_add_path /opt/mytool/bin/mytool
```

it may tell you "Skipping path because it is a file instead of a
directory:".

The check isn't perfect, it goes through status current-command and
isatty, but it should be good for most cases (and be false in config.fish).

doc_src/cmds/fish_add_path.rst

@@ -52,7 +52,8 @@ Options
     Move already-included directories to the place they would be added - by default they would be left in place and not added again.
 
 **-v** or **--verbose**
-    Print the :doc:`set <set>` command used.
+    Print the :doc:`set <set>` command used, and some more warnings, like when a path is skipped because it doesn't exist or is not a directory.
+    Verbose mode is automatically enabled when fish_add_path is used interactively and the output goes to the terminal.
 
 **-n** or **--dry-run**
     Print the ``set`` command that would be used without executing it.

share/functions/fish_add_path.fish

@@ -36,6 +36,11 @@ function fish_add_path --description "Add paths to the PATH"
     set -l mode $_flag_prepend $_flag_append
     set -q mode[1]; or set mode -p
 
+    # Enable verbose mode if we're interactively used
+    status current-command | string match -rq '^fish_add_path$'
+    and isatty stdout
+    and set -l _flag_verbose yes
+
     # To keep the order of our arguments, go through and save the ones we want to keep.
     set -l newpaths
     set -l indexes