(bug 35749) Update checkSyntax.php to use Git

(bug 35749) Update checkSyntax.php to use Git

checkSyntax.php now uses git commands to retrieve a list of modified
files instead of the svn command-line client.

The script now considers any files that have been changed since
the nearest common ancestor to the master branch to be "modified",
as it is probably desirable to include files modified in previous
commits on the same local topic branch that have not yet made it
into master.

Change-Id: I0d4a02d4591242a9fd45291f5c4230c535e861b3