Tap: allow enumerate non-git taps

Also add a `Tap#git?` method to indicate whether the tap is a git
repository.

Closes Homebrew/homebrew#42264.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/cmd/update.rb

@@ -27,7 +27,7 @@ module Homebrew
     # this procedure will be removed in the future if it seems unnecessasry
     rename_taps_dir_if_necessary
 
-    Tap.each do |tap|
+    Tap.select(&:git?).each do |tap|
       tap.path.cd do
         updater = Updater.new(tap.path)
 

Library/Homebrew/tap.rb

@@ -37,7 +37,7 @@ class Tap
   # e.g. `https://github.com/user/homebrew-repo`
   def remote
     @remote ||= if installed?
-      if (@path/".git").exist?
+      if git?
         @path.cd do
           Utils.popen_read("git", "config", "--get", "remote.origin.url").chomp
         end
@@ -49,6 +49,11 @@ class Tap
     end
   end
 
+  # True if this {Tap} is a git repository.
+  def git?
+    (@path/".git").exist?
+  end
+
   def to_s
     name
   end
@@ -117,12 +122,10 @@ class Tap
 
     TAP_DIRECTORY.subdirs.each do |user|
       user.subdirs.each do |repo|
-        if (repo/".git").directory?
         yield new(user.basename.to_s, repo.basename.to_s.sub("homebrew-", ""))
       end
     end
   end
-  end
 
   # an array of all installed {Tap} names.
   def self.names