Git build should record the actual tag, not the latest one

Try to build older jtreg5.1-b01 from the current workspace, like this:

$ git checkout jtreg5.1-b01
$ rm -rf build; sh make/build-all.sh ~/Install/jdk8u252-ea/
...

$ build/images/jtreg/bin/jtreg -version
jtreg, version jtreg6 dev jtreg6

$ cat build/images/jtreg/release
JTREG_VERSION=jtreg6 jtreg6
BUILD_DATE=June 28, 2021

Note it reports jtreg6! This would then fail when JDK tries to poll the JTReg version for internal checks.

This happens because the build polls last Git tag, regardless of whether the current HEAD is at previous changeset. I think we are better off using "git describe".

diff --git a/make/build-all.sh b/make/build-all.sh
index 150e58d..46c4ace 100644
--- a/make/build-all.sh
+++ b/make/build-all.sh
@@ -84,7 +84,7 @@ get_root() {
 get_tag_info() {
    case $SCM_TYPE in
        HG) hg tags | grep jtreg | head -1 ;;
- GIT) git tag | grep jtreg | tail -1 ;;
+ GIT) git describe ;;
        *) echo "Error: unknown SCM" >&2 ; exit 1 ;;
    esac
 }

After this:

$ cat build/images/jtreg/release
JTREG_VERSION=5.1 b01
BUILD_DATE=June 28, 2021

$ build/images/jtreg/bin/jtreg -version
jtreg, version 5.1 dev b01

While this patch would not help building the older JTRegs without rewriting the repository history, it would be good to get in fixed for future JTRegs. This patch can be used separately to build older JTRegs.