Improve generation of version information from the git tree

Detect whether the tree is dirty and append a "-dirty" indication to the
git commit (common practice with other repos, e.g. kernel, docker.)

Properly handle the case where a git tree is not found (e.g. building
from archive.)

In the sed expression, look for the variable to be updated
(commitFromGit) instead of hardcoding a line number.

Tested:

- Built from a dirty tree:
    $ output/go/bin/kubelet -version
    Kubernetes version 0.1, build 2d784c684c75-dirty

- Built from a clean tree:
    $ output/go/bin/kubelet -version
    Kubernetes version 0.1, build 505f23a31172

- Built from an archive:
    $ hack/build-go.sh
    WARNING: unable to find git commit, falling back to commitFromGit = `(none)`
    $ output/go/bin/kubelet -version
    Kubernetes version 0.1, build (none)

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

hack/version-gen.sh

@@ -14,10 +14,28 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+set -o errexit
+set -o nounset
+set -o pipefail
+
+topdir=$(dirname "$0")/..
+cd "${topdir}"
+
 # TODO: when we start making tags, switch to git describe?
-desc=$(git rev-list --abbrev-commit --max-count=1 HEAD)
-tab=$'\t'
-script="22s/.*/${tab}commitFromGit = \`${desc}\`/"
-infile="$(dirname $0)/../pkg/version/template.go"
-outfile="$(dirname $0)/../pkg/version/autogenerated.go"
-sed "${script}" "${infile}" > "${outfile}"
+if git_commit=$(git rev-parse --short "HEAD^{commit}" 2>/dev/null); then
+  # Remove any invalid characters that might confuse "sed".
+  git_commit=${git_commit//[^a-f0-9]/}
+
+  # Check if the tree is dirty.
+  if ! dirty_tree=$(git status --porcelain) || [[ -n "${dirty_tree}" ]]; then
+    git_commit="${git_commit}-dirty"
+  fi
+else
+  git_commit="(none)"
+  echo "WARNING: unable to find git commit, falling back to commitFromGit = \`${git_commit}\`" >&2
+fi
+
+# TODO: Instead of using an autogenerated file, we could pass this variable
+# to the source through Go's -X ldflag.
+sed "s/@@GIT_COMMIT@@/${git_commit}/g" \
+  pkg/version/template.go.tmpl >pkg/version/autogenerated.go