redhat/Makefile: fix access issue on shared build system

Bugzilla: INTERNAL
Upstream-status: RHEL-only
Tested: local make dist-release-finish + check commit content

In a shared build systems the hardcoded file /tmp/rtkcommit is a problem
when different users build kernel-rt before deleting the file or
rebooting the system. Since the file has its name hardcoded, the first
user creating it will have write access to it, while the second faces
-EACCESS.

Instead of using a hardcoded temporary filename, directly call git
using the -m flag to pass the commit log text, preventing any
temporary file.

Suggested-by: Frantisek Hrbata fhrbata@redhat.com
Signed-off-by: Bruno Meneguele bmeneg@redhat.com