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>