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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: parkr

contrib/persistent-https/Makefile

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-BUILD_LABEL=$(shell date +"%s")
+BUILD_LABEL=$(shell cut -d" " -f3 ../../GIT-VERSION-FILE)
 TAR_OUT=$(shell go env GOOS)_$(shell go env GOARCH).tar.gz
 
 all: git-remote-persistent-https git-remote-persistent-https--proxy \