The bash script now adds a #define if the current checkout is
exactly on a tag, and it leaves all #defines empty if building
from a tarball.
The batch file still needs to be updated.
It was previously working in a false-positive state, where it would behave as
expected if you ran "make" from within a subdirectory of the git repo. If you
ran make anywhere else, at least diff-index would break.
Change -C to --work-dir to work on earlier versions of git.
Add "+" to the revision number if there are uncommitted changes.
Only rewrite the git_info.h if there are changes, so that make doesn't think the file has changed.
