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

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

Commit 334d28ae 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>