tikv_jemalloc_ctl::version can show the version of the repository using jemalloc

[espindola]

If one creates a simple program with just

use tikv_jemalloc_ctl::version;
fn main() {
    println!("jemalloc version {}", version::read().unwrap());
}

and place it in a git repo and tag a revision with:

$ git tag -a 1.2.3 -m foo

The program will print

jemalloc version 1.2.3-0-....

Because the git repo that configure sees is the one with the test program.

I will try to write a PR.