Makefile: drop dependency between git-instaweb and gitweb

The rule for "git-instaweb" depends on "gitweb". This makes
no sense, because:

  1. git-instaweb has no build-time dependency on gitweb; it
     is a run-time dependency

  2. gitweb is a directory that we want to recursively make
     in. As a result, its recipe is marked .PHONY, which
     causes "make" to rebuild git-instaweb every time it is
     run.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1722,7 +1722,7 @@ GIT-PERL-DEFINES: FORCE
 gitweb:
 	$(QUIET_SUBDIR0)gitweb $(QUIET_SUBDIR1) all
 
-git-instaweb: git-instaweb.sh gitweb GIT-SCRIPT-DEFINES
+git-instaweb: git-instaweb.sh GIT-SCRIPT-DEFINES
 	$(QUIET_GEN)$(cmd_munge_script) && \
 	chmod +x $@+ && \
 	mv $@+ $@