In 20fc9bc (Set HTTP user agent to git/GIT_VERSION, 2006-04-04),
http.o started recording GIT_VERSION, but http.o wasn't added
to the list of files that depends on GIT-VERSION-FILE.
Fix this, so mofications to GIT-VERSION-FILE will result in an
updated user-agent string.
Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>