[ErrorReport] Include git information in stack

Closes https://github.com/CocoaPods/CocoaPods/issues/2678.

lib/cocoapods/user_interface/error_report.rb

@@ -29,6 +29,7 @@ module Pod
     RubyGems : #{Gem::VERSION}
         Host : #{host_information}
        Xcode : #{xcode_information}
+         Git : #{git_information}
 Ruby lib dir : #{RbConfig::CONFIG['libdir']}
 Repositories : #{repo_information.join("\n               ")}
 ```
@@ -105,6 +106,10 @@ EOS
           "#{version} (#{build})"
         end
 
+        def git_information
+          `git --version`.strip.split("\n").first
+        end
+
         def installed_plugins
           CLAide::Command::PluginsHelper.specifications.
             reduce({}) { |hash, s| hash.tap { |h| h[s.name] = s.version.to_s } }

spec/unit/user_interface/error_report_spec.rb

@@ -35,6 +35,7 @@ module Pod
     RubyGems : #{Gem::VERSION}
         Host : :host_information
        Xcode : :xcode_information
+         Git : :git_information
 Ruby lib dir : #{RbConfig::CONFIG['libdir']}
 Repositories : repo_1
                repo_2
@@ -85,6 +86,7 @@ EOS
 EOS
         @report.stubs(:host_information).returns(':host_information')
         @report.stubs(:xcode_information).returns(':xcode_information')
+        @report.stubs(:git_information).returns(':git_information')
         @report.stubs(:repo_information).returns(%w(repo_1 repo_2))
         @report.stubs(:installed_plugins).returns('cocoapods' => Pod::VERSION,
                                                   'cocoapods-core' => Pod::VERSION,