http-push: add regression tests

http-push: add regression tests

http-push tests require a web server with WebDAV support.

This commit introduces a HTTPD test library, which can be configured using
the following environment variables.

GIT_TEST_HTTPD		enable HTTPD tests
LIB_HTTPD_PATH		web server path
LIB_HTTPD_MODULE_PATH	web server modules path
LIB_HTTPD_PORT		listening port
LIB_HTTPD_DAV		enable DAV
LIB_HTTPD_SVN		enable SVN
LIB_HTTPD_SSL		enable SSL

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>