Add a method for installing fish function files

Fish shell allows third-party software vendors to put their own function files in a directory for their software.
For brew installed Fish shell, this is /usr/local/share/fish/vendor_functions.d

Library/Homebrew/caveats.rb

@@ -19,6 +19,7 @@ class Caveats
     caveats << bash_completion_caveats
     caveats << zsh_completion_caveats
     caveats << fish_completion_caveats
+    caveats << fish_function_caveats
     caveats << plist_caveats
     caveats << python_caveats
     caveats << app_caveats
@@ -99,6 +100,17 @@ class Caveats
     EOS
   end
 
+  def fish_function_caveats
+    return unless keg
+    return unless keg.fish_functions_installed?
+    return unless which("fish")
+
+    <<-EOS.undent
+      fish functions have been installed to:
+        #{HOMEBREW_PREFIX}/share/fish/vendor_functions.d
+    EOS
+  end
+
   def python_caveats
     return unless keg
     return unless keg.python_site_packages_installed?

Library/Homebrew/formula.rb

@@ -778,6 +778,14 @@ class Formula
     HOMEBREW_PREFIX+"var"
   end
 
+  # The directory where the formula's fish function files should be
+  # installed.
+  # This is symlinked into `HOMEBREW_PREFIX` after installation or with
+  # `brew link` for formulae that are not keg-only.
+  def fish_function
+    share+"fish/vendor_functions.d"
+  end
+
   # The directory where the formula's Bash completion files should be
   # installed.
   # This is symlinked into `HOMEBREW_PREFIX` after installation or with

Library/Homebrew/keg.rb

@@ -303,6 +303,11 @@ class Keg
     dir && dir.directory? && !dir.children.empty?
   end
 
+  def fish_functions_installed?
+    dir = path.join("share", "fish", "vendor_functions.d")
+    dir && dir.directory? && !dir.children.empty?
+  end
+
   def plist_installed?
     !Dir["#{path}/*.plist"].empty?
   end