Tell a server that protocol v1 can be used by sending the http header
'Git-Protocol' with 'version=1' indicating this.

Also teach the apache http server to pass through the 'Git-Protocol'
header as an environment variable 'GIT_PROTOCOL'.
Signed-off-by: NBrandon Williams <bmwill@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>