contrib/persistent-https: update ldflags syntax for Go 1.7+

contrib/persistent-https: update ldflags syntax for Go 1.7+

Running `make all` in `contrib/persistent-https` results in a
failure on Go 1.7 and above.

Specifically, the error is:

    go build -o git-remote-persistent-https \
   -ldflags "-X main._BUILD_EMBED_LABEL 1468613136"
    # _/Users/parkr/github/git/contrib/persistent-https
    /usr/local/Cellar/go/1.7rc1/libexec/pkg/tool/darwin_amd64/link: -X
flag requires argument of the form importpath.name=value
    make: *** [git-remote-persistent-https] Error 2

This `name=value` syntax for the -X flag was introduced in Go v1.5
(released Aug 19, 2015):

 - release notes: https://golang.org/doc/go1.5#link
 - commit: https://github.com/golang/go/commit/12795c02f3d6fc54ece09a86e70aaa40a94d5131

In Go v1.7, support for the old syntax was removed:

 - release notes: https://tip.golang.org/doc/go1.7#compiler
 - commit: https://github.com/golang/go/commit/51b624e6a29b135ce0fadb22b678acf4998ff16f

Add '=' between the symbol and its value for recent versions of Go,
while leaving it out for older ones.

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