Bazel: Migrate workspace status script to python

Migrate tools/workspace-status.sh script to Python to ensure better
compatibility with other operating systems, as suggested in: [1].

[1] https://github.com/bazelbuild/bazel/issues/5958

Change-Id: I20ec325d610353309a40fc85132382004bc61bd0
diff --git a/.bazelrc b/.bazelrc
index 4ed16cf..3ae03ff 100644
--- a/.bazelrc
+++ b/.bazelrc
@@ -1,2 +1,2 @@
-build --workspace_status_command=./tools/workspace-status.sh
+build --workspace_status_command="python ./tools/workspace_status.py"
 test --build_tests_only
diff --git a/tools/workspace-status.sh b/tools/workspace-status.sh
deleted file mode 100755
index 10ba369..0000000
--- a/tools/workspace-status.sh
+++ /dev/null
@@ -1,17 +0,0 @@
-#!/bin/bash
-
-# This script will be run by bazel when the build process starts to
-# generate key-value information that represents the status of the
-# workspace. The output should be like
-#
-# KEY1 VALUE1
-# KEY2 VALUE2
-#
-# If the script exits with non-zero code, it's considered as a failure
-# and the output will be discarded.
-
-function rev() {
-  cd $1; git describe --always --match "v[0-9].*" --dirty
-}
-
-echo STABLE_BUILD_SLACK-INTEGRATION_LABEL $(rev .)
diff --git a/tools/workspace_status.py b/tools/workspace_status.py
new file mode 100644
index 0000000..5e76924
--- /dev/null
+++ b/tools/workspace_status.py
@@ -0,0 +1,31 @@
+#!/usr/bin/env python
+
+# This script will be run by bazel when the build process starts to
+# generate key-value information that represents the status of the
+# workspace. The output should be like
+#
+# KEY1 VALUE1
+# KEY2 VALUE2
+#
+# If the script exits with non-zero code, it's considered as a failure
+# and the output will be discarded.
+
+from __future__ import print_function
+import subprocess
+import sys
+
+CMD = ['git', 'describe', '--always', '--match', 'v[0-9].*', '--dirty']
+
+
+def revision():
+    try:
+        return subprocess.check_output(CMD).strip().decode("utf-8")
+    except OSError as err:
+        print('could not invoke git: %s' % err, file=sys.stderr)
+        sys.exit(1)
+    except subprocess.CalledProcessError as err:
+        print('error using git: %s' % err, file=sys.stderr)
+        sys.exit(1)
+
+
+print("STABLE_BUILD_SLACK-INTEGRATION_LABEL %s" % revision())