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>

Makefile

@@ -1606,14 +1606,8 @@ $(patsubst %.py,%,$(SCRIPT_PYTHON)): % : %.py
 	INSTLIBDIR=`MAKEFLAGS= $(MAKE) -C git_remote_helpers -s \
 		--no-print-directory prefix='$(prefix_SQ)' DESTDIR='$(DESTDIR_SQ)' \
 		instlibdir` && \
-	sed -e '1{' \
-	    -e '	s|#!.*python|#!$(PYTHON_PATH_SQ)|' \
-	    -e '}' \
-	    -e 's|^import sys.*|&; \\\
-	           import os; \\\
-	           sys.path[0] = os.environ.has_key("GITPYTHONLIB") and \\\
-	                         os.environ["GITPYTHONLIB"] or \\\
-	                         "@@INSTLIBDIR@@"|' \
+	sed -e '1s|#!.*python|#!$(PYTHON_PATH_SQ)|' \
+	    -e 's|\(os\.getenv("GITPYTHONLIB"\)[^)]*)|\1,"@@INSTLIBDIR@@")|' \
 	    -e 's|@@INSTLIBDIR@@|'"$$INSTLIBDIR"'|g' \
 	    $@.py >$@+ && \
 	chmod +x $@+ && \

git-remote-testgit.py

@@ -2,6 +2,8 @@
 
 import hashlib
 import sys
+import os
+sys.path.insert(0, os.getenv("GITPYTHONLIB","."))
 
 from git_remote_helpers.util import die, debug, warn
 from git_remote_helpers.git.repo import GitRepo