Formula#file_modfiled?: remove unnecessary git dir check

It's already covered by `$?.exitstatus == 0` in below.

1 files changed, 1 insertions, 3 deletions

diff --git a/Library/Homebrew/formula.rb b/Library/Homebrew/formula.rb
index 0e3830e1f..7ec035dd7 100644
--- a/Library/Homebrew/formula.rb
+++ b/Library/Homebrew/formula.rb
@@ -765,10 +765,8 @@ class Formula
   end
 
   def file_modified?
-    # New Homebrew installs may not be a git repo yet
-    return false unless File.exist? "#{HOMEBREW_PREFIX}/.git"
-
     git = which("git")
+
     # git isn't installed by older Xcodes
     return false if git.nil?