Makefile: Remove usage of deprecated Python "has_key" method

"has_key" is a deprecated dictionary method in Python 2.6+.
Simplify the sys.path manipulation for installed scripts by
passing a default value to os.getenv() that takes a default
value to be used when the environment variable is missing.

SCRIPT_PYTHON is currently empty but this future-proofs us.
It also fixes things for users who maintain local git forks
with their own SCRIPT_PYTHON additions.

Old code replaced the first element of sys.path[] which is
typically '' (i.e. import library files relative to the script).
It is safer to prepend the extra library path instead.

Signed-off-by: David Aguilar <davvid@gmail.com>
Acked-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1557,9 +1557,8 @@ $(patsubst %.py,%,$(SCRIPT_PYTHON)): % : %.py
 	    -e '}' \
 	    -e 's|^import sys.*|&; \\\
 	           import os; \\\
-	           sys.path[0] = os.environ.has_key("GITPYTHONLIB") and \\\
-	                         os.environ["GITPYTHONLIB"] or \\\
-	                         "@@INSTLIBDIR@@"|' \
+	           sys.path.insert(0, os.getenv("GITPYTHONLIB",\
+	                                        "@@INSTLIBDIR@@"));|' \
 	    -e 's|@@INSTLIBDIR@@|'"$$INSTLIBDIR"'|g' \
 	    $@.py >$@+ && \
 	chmod +x $@+ && \