Validate the output of xcrun more thoroughly

Noticed when installing Homebrew on a fresh install of 10.7 printed
this:

  -e:204: command not found: Error: /usr/bin/xcode-select returned unexpected error. --version

1 files changed, 2 insertions, 2 deletions

diff --git a/install b/install
index dae9783b5..0d131ca44 100755
--- a/install
+++ b/install
@@ -80,8 +80,8 @@ def git
   elsif Kernel.system '/usr/bin/which -s git'
     'git'
   else
-    s = `xcrun -find git 2>/dev/null`.chomp
-    s if $? and not s.empty?
+    exe = `xcrun -find git 2>/dev/null`.chomp
+    exe if $? && $?.success? && !exe.empty? && File.executable?(exe)
   end
 
   return unless @git