Add support for fish shell completions

* Library/Homebrew/caveats.rb: add caveats comparable to other shells
* Library/Homebrew/formula.rb: define completion directory along the
  conventions explained in the fish documentation for
  $fish_complete_path
* Library/Homebrew/keg.rb: add fish shell to check function for
  installed completions

Closes Homebrew/homebrew#39828.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/caveats.rb

@@ -12,6 +12,7 @@ class Caveats
     caveats << f.keg_only_text if f.keg_only? && f.respond_to?(:keg_only_text)
     caveats << bash_completion_caveats
     caveats << zsh_completion_caveats
+    caveats << fish_completion_caveats
     caveats << plist_caveats
     caveats << python_caveats
     caveats << app_caveats
@@ -46,6 +47,14 @@ class Caveats
     end
   end
 
+  def fish_completion_caveats
+    if keg and keg.completion_installed? :fish and which("fish") then <<-EOS.undent
+      fish completion has been installed to:
+        #{HOMEBREW_PREFIX}/share/fish/vendor_completions.d
+      EOS
+    end
+  end
+
   def python_caveats
     return unless keg
     return unless keg.python_site_packages_installed?

Library/Homebrew/formula.rb

@@ -422,6 +422,12 @@ class Formula
   # `brew link` for formulae that are not keg-only.
   def zsh_completion;  share+'zsh/site-functions'        end
 
+  # The directory where the formula's fish completion 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_completion; share+'fish/vendor_completions.d' end
+
   # The directory used for as the prefix for {#etc} and {#var} files on
   # installation so, despite not being in `HOMEBREW_CELLAR`, they are installed
   # there after pouring a bottle.

Library/Homebrew/keg.rb

@@ -222,6 +222,7 @@ class Keg
     dir = case shell
           when :bash then path.join("etc", "bash_completion.d")
           when :zsh  then path.join("share", "zsh", "site-functions")
+          when :fish then path.join("share", "fish", "vendor_completions.d")
           end
     dir && dir.directory? && dir.children.any?
   end