Update the commit-msg hook to the version from Gerrit 2.5.2

Change-Id: I00760fe55a0e1b61375a378c05f263e7bc857ca0

hooks/commit-msg

@@ -1,5 +1,5 @@
 #!/bin/sh
-# From Gerrit Code Review 2.5-rc0
+# From Gerrit Code Review 2.5.2
 #
 # Part of Gerrit Code Review (http://code.google.com/p/gerrit/)
 #
@@ -18,6 +18,8 @@
 # limitations under the License.
 #
 
+unset GREP_OPTIONS
+
 CHANGE_ID_AFTER="Bug|Issue"
 MSG="$1"