__fish_complete_directories: Remove --foo= from token

Otherwise this would complete

`git --exec-path=foo`, by running `complete -C"'' --exec-path=foo"`,

which would print "--exec-path=foo", and so it would end as

`git --exec-path=--exec-path=foo` because the "replaces token" bit was
lost.

I'm not sure how to solve it cleanly - maybe an additional option to
`complete`?

Anyway, for now this
Fixes #9538.

share/functions/__fish_complete_directories.fish

@@ -9,7 +9,11 @@ function __fish_complete_directories -d "Complete directory prefixes" --argument
     end
 
     if not set -q comp[1]
-        set comp (commandline -ct)
+        # No token given, so we use the current commandline token.
+        # If we have a --name=val option, we need to remove it,
+        # or the complete -C below would keep it, and then whatever complete
+        # called us would add it again (assuming it's in the current token)
+        set comp (commandline -ct | string replace -r -- '^-[^=]*=' '' $comp)
     end
 
     # HACK: We call into the file completions by using an empty command

tests/checks/complete_directories.fish

@@ -1,4 +1,5 @@
-#RUN: %fish %s
+#RUN: %fish --interactive %s
+# ^ interactive so we can do `complete`
 mkdir -p __fish_complete_directories/
 cd __fish_complete_directories
 mkdir -p test/buildroot
@@ -25,3 +26,7 @@ __fish_complete_directories test/data/
 __fish_complete_directories test/data/abc 'abc dirs'
 #CHECK: test/data/abc/	abc dirs
 #CHECK: test/data/abcd/	abc dirs
+
+complete -c mydirs -l give-me-dir -a '(__fish_complete_directories)'
+complete -C'mydirs --give-me-dir='
+#CHECK: --give-me-dir=test/{{\t}}Directory