Use :pgroup option when starting subprocesses

This makes it easier and faster to kill spawned git processes.
Repetitive `pgrep` command for collecting PIDs is no longer required.
Unfortunately, :pgroup option is not supported on Ruby 1.8 so we can't
remove the old implementation of `killall`.

plug.vim

@@ -1003,6 +1003,22 @@ function! s:update_ruby()
     %["#{arg.gsub('"', '\"')}"]
   end
 
+  require 'rubygems'
+  require 'thread'
+  require 'fileutils'
+  require 'timeout'
+
+  if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('1.9')
+    def popen cmd
+      IO.popen(cmd, :pgroup => true)
+    end
+    def killall pid
+      Process.kill 'TERM', - pid rescue nil
+    end
+  else
+    def popen cmd
+      IO.popen(cmd)
+    end
     def killall pid
       pids = [pid]
       unless `which pgrep 2> /dev/null`.empty?
@@ -1016,10 +1032,7 @@ function! s:update_ruby()
       end
       pids.each { |pid| Process.kill 'TERM', pid.to_i rescue nil }
     end
-
+  end
-  require 'thread'
-  require 'fileutils'
-  require 'timeout'
   running = true
   iswin = VIM::evaluate('s:is_win').to_i == 1
   pull  = VIM::evaluate('s:update.pull').to_i == 1
@@ -1086,7 +1099,7 @@ function! s:update_ruby()
           File.unlink tmp rescue nil
         end
       else
-        fd = IO.popen(cmd).extend(PlugStream)
+        fd = popen(cmd).extend(PlugStream)
         first_line = true
         log_prob = 1.0 / nthr
         while line = Timeout::timeout(timeout) { fd.get_line }