Add SHA-1 hash of content of SUT source file to XML report

[sebastianbergmann]

Currently we have this for the individual XML files we generate for the SUT source files:

<phpunit xmlns="https://schema.phpunit.de/coverage/1.0">
  <file name="BankAccount.php" path="%e">
   <!-- ... -->

This should be changed to

<phpunit xmlns="https://schema.phpunit.de/coverage/1.0">
  <file name="BankAccount.php" path="%e" hash="<SHA-1 of contents of file referenced by path attribute>">
   <!-- ... -->

And in the index.xml we have

<directory name="%s">
 <!-- ... -->
 <file name="BankAccount.php" href="BankAccount.php.xml">

This should be changed to

<directory name="%s">
 <!-- ... -->
 <file name="BankAccount.php" href="BankAccount.php.xml" hash="<SHA-1 of contents of file referenced by path attribute>">

CC @theofidry