Set gitMajor and gitMinor from hack/build-go.sh

Set the values of major and minor version based on the output of the
`git describe` command, which uses annotated tags as source of
versioning information.

Minor will get a "+" appended whenever the annotated tag does not match
the tree exactly (including when the tree is dirty.) So that only
official releases will have a "bare" minor version, all others will have
a "+" to indicate the binaries contain changes from the released version
in minor.

(This is similar to how versions of development builds of the Linux
kernel work.)

Tested:
- With no annotated tags:
  $ hack/build-go.sh
  $ _output/go/bin/kubecfg -version=raw
  version.Info{Major:"0", Minor:"1+", GitVersion:"v0.1+", GitCommit:"7d29873bdee87efacaace30ab3602297cacf1b4f", GitTreeState:"clean"}

- Tagging current version on a clean git tree:
  $ git tag -a -m 'Test tag v2.3' v2.3
  $ hack/build-go.sh
  $ _output/go/bin/kubecfg -version=raw
  version.Info{Major:"2", Minor:"3", GitVersion:"v2.3", GitCommit:"7d29873bdee87efacaace30ab3602297cacf1b4f", GitTreeState:"clean"}

- Tagging current version on a dirty git tree:
  $ git tag -a -m 'Test tag v2.3' v2.3
  $ touch test.txt  # this is enough to mark the tree as dirty
  $ hack/build-go.sh
  $ _output/go/bin/kubecfg -version=raw
  version.Info{Major:"2", Minor:"3+", GitVersion:"v2.3-dirty", GitCommit:"7d29873bdee87efacaace30ab3602297cacf1b4f", GitTreeState:"dirty"}

- Tagging a previous version on a clean tree:
  $ git tag -a -m 'Test tag v2.3' v2.3 HEAD~5
  $ hack/build-go.sh
  $ _output/go/bin/kubecfg -version=raw
  version.Info{Major:"2", Minor:"3+", GitVersion:"v2.3-6-g7d29873bdee87e", GitCommit:"7d29873bdee87efacaace30ab3602297cacf1b4f", GitTreeState:"clean"}

Signed-off-by: Filipe Brandenburger <filbranden@google.com>

hack/config-go.sh

@@ -54,6 +54,21 @@ kube::version_ldflags() {
           git_version+="-dirty"
         fi
         ldflags+=(-X "${KUBE_GO_PACKAGE}/pkg/version.gitVersion" "${git_version}")
+
+        # Try to match the "git describe" output to a regex to try to extract
+        # the "major" and "minor" versions and whether this is the exact tagged
+        # version or whether the tree is between two tagged versions.
+        if [[ "${git_version}" =~ ^v([0-9]+)\.([0-9]+)([.-].*)?$ ]]; then
+          git_major=${BASH_REMATCH[1]}
+          git_minor=${BASH_REMATCH[2]}
+          if [[ -n "${BASH_REMATCH[3]}" ]]; then
+            git_minor+="+"
+          fi
+          ldflags+=(
+            -X "${KUBE_GO_PACKAGE}/pkg/version.gitMajor" "${git_major}"
+            -X "${KUBE_GO_PACKAGE}/pkg/version.gitMinor" "${git_minor}"
+          )
+        fi
       fi
     fi