Makefile: Simplify handling of python scripts

Makefile: Simplify handling of python scripts

The sed script intended to add a standard opening to python scripts
was non-compatible and overly complex.  Simplifying it down to a set
of one-liners removes the compatibility issues of newlines.  Moving
the environment alterations from the Makefile to the python scripts
makes also makes the scripts easier to run in-place.

Specifically, the new sed script:

 - Alters the shebang line to use the configured Python.
 - Alters any os.getenv("GITPYTHONLIB") calls to use @@INSTLIBDIR@@ as the
   default.  This will replace any existing default or add a default if
   none is provided.
 - Replaces the @@INSTLIBDIR@@ placeholder with the directory git installs
   its python libraries to.

The last two steps could be combined into a single step, but is left
separate in case someone has another need for @@INSTLIBDIR@@ in their
script.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Brian Gernhardt <brian@gernhardtsoftware.com>
Acked-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>