Allow installing in the traditional way

In an earlier commit c70a8d98 (Makefile: Do not install a copy of 'git' in
$(gitexecdir), 2008-07-21), we tried to avoid installing two git, one in
/usr/bin/git and the other in /usr/libexec/git-core/git.  It mistakenly
removed the only copy of git when gitexecdir and bindir are set to the
same directory, i.e. the traditional layout.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -1362,7 +1362,10 @@ endif
 		cp "$$bindir/git$X" "$$execdir/git$X"; \
 	fi && \
 	{ $(foreach p,$(BUILT_INS), $(RM) "$$execdir/$p" && ln "$$execdir/git$X" "$$execdir/$p" ;) } && \
-	$(RM) "$$execdir/git$X" && \
+	if test "z$$bindir" != "z$$execdir"; \
+	then \
+		$(RM) "$$execdir/git$X"; \
+	fi && \
 	./check_bindir "z$$bindir" "z$$execdir" "$$bindir/git-add$X"
 
 install-doc: