KegOnlyReason: print only the explanation if there’s one

Closes Homebrew/homebrew#42073.

Signed-off-by: Baptiste Fontaine <batifon@yahoo.fr>

Library/Homebrew/formula_support.rb

@@ -23,28 +23,23 @@ class KegOnlyReason
   end
 
   def to_s
+    return @explanation unless @explanation.empty?
     case @reason
     when :provided_by_osx then <<-EOS
 OS X already provides this software and installing another version in
 parallel can cause all kinds of trouble.
-
-#{@explanation}
 EOS
     when :shadowed_by_osx then <<-EOS
 OS X provides similar software, and installing this software in
 parallel can cause all kinds of trouble.
-
-#{@explanation}
 EOS
     when :provided_pre_mountain_lion then <<-EOS
 OS X already provides this software in versions before Mountain Lion.
-
-#{@explanation}
 EOS
     when :provided_until_xcode43
-      "Xcode provides this software prior to version 4.3.\n\n#{@explanation}"
+      "Xcode provides this software prior to version 4.3."
     when :provided_until_xcode5
-      "Xcode provides this software prior to version 5.\n\n#{@explanation}"
+      "Xcode provides this software prior to version 5."
     else
       @reason
     end.strip

Library/Homebrew/test/test_formula_support.rb

+require "testing_env"
+
+class KegOnlyReasonTests < Homebrew::TestCase
+  def test_to_s_explanation
+    r = KegOnlyReason.new :provided_by_osx, "test"
+    assert_equal "test", r.to_s
+  end
+
+  def test_to_s_no_explanation
+    r = KegOnlyReason.new :provided_by_osx, ""
+    assert_match(/^OS X already provides/, r.to_s)
+  end
+end