Include `-dirty` suffix in version info when building on a dirty tree

Ensure the output of `git describe` will include the `-dirty` suffix
when building on a tree with modified files in it.

Tested:
- ...

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
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>
Merge pull request #1122 from filbranden/version_major_minor1

Push major and minor versions from git