Makefile: rebuild git.o on version change, clean up git$X flags

Commit 334d28a factored out git.o as an intermediate stage between
git.c and git$X.  However:

- It left some no-longer-relevant flags in the rule for git$X.

- It failed to replace git$X with git.o in the list of files that
  record GIT_VERSION.  This broke incorporation of a changed
  GIT_VERSION into git$X because, when GIT_VERSION changes, git.o isn't
  remade and git$X is relinked from the git.o that still contains the
  old GIT_VERSION.

This patch removes the irrelevant flags and fixes incorporation of a
changed GIT_VERSION into git$X.

Signed-off-by: Matt McCutchen <hashproduct@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -753,8 +753,7 @@ git.o: git.c common-cmds.h GIT-CFLAGS
 		$(ALL_CFLAGS) -c $(filter %.c,$^)
 
 git$X: git.o $(BUILTIN_OBJS) $(GITLIBS)
-	$(QUIET_LINK)$(CC) -DGIT_VERSION='"$(GIT_VERSION)"' \
-		$(ALL_CFLAGS) -o $@ $(filter %.c,$^) git.o \
+	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ git.o \
 		$(BUILTIN_OBJS) $(ALL_LDFLAGS) $(LIBS)
 
 help.o: common-cmds.h
@@ -857,7 +856,7 @@ configure: configure.ac
 	rm -f $<+
 
 # These can record GIT_VERSION
-git$X git.spec \
+git.o git.spec \
 	$(patsubst %.sh,%,$(SCRIPT_SH)) \
 	$(patsubst %.perl,%,$(SCRIPT_PERL)) \
 	: GIT-VERSION-FILE