contrib/persistent-https: use Git version for build label

contrib/persistent-https: use Git version for build label

The previous method simply used the UNIX timestamp of when the binary was
built as its build label.

    $ make && ./git-remote-persistent-http -print_label
    1469061546

This patch aims to align the label for this binary with the Git version
contained in the GIT-VERSION-FILE. This gives a better sense of the version
of the binary as it can be mapped to a particular revision or release of
Git itself. For example:

    $ make && ./git-remote-persistent-http -print_label
    2.9.1.275.g75676c8

Discussion of this patch is available on a related thread in the mailing
list surrounding this package called "contrib/persistent-https: update
ldflags syntax for Go 1.7+". The gmane.org link is:
http://article.gmane.org/gmane.comp.version-control.git/299653/

Signed-off-by: Parker Moore <parkrmoore@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>