Use install_venv_common.py from oslo.

This syncs install_venv_common.py from oslo and reworks the
tools/install_venv.py script to use the new library.

Change-Id: I3426a7f51b0018e074cc6f4b1d70b38e52464a38

openstackclient/openstack/common/setup.py

@@ -258,6 +258,22 @@ def get_cmdclass():
     return cmdclass
 
 
+def _get_revno():
+    """Return the number of commits since the most recent tag.
+
+    We use git-describe to find this out, but if there are no
+    tags then we fall back to counting commits since the beginning
+    of time.
+    """
+    describe = _run_shell_command("git describe --always")
+    if "-" in describe:
+        return describe.rsplit("-", 2)[-2]
+
+    # no tags found
+    revlist = _run_shell_command("git rev-list --abbrev-commit HEAD")
+    return len(revlist.splitlines())
+
+
 def get_version_from_git(pre_version):
     """Return a version which is equal to the tag that's on the current
     revision if there is one, or tag plus number of additional revisions
@@ -271,9 +287,7 @@ def get_version_from_git(pre_version):
                     throw_on_error=True).replace('-', '.')
             except Exception:
                 sha = _run_shell_command("git log -n1 --pretty=format:%h")
-                describe = _run_shell_command("git describe --always")
+                return "%s.a%s.g%s" % (pre_version, _get_revno(), sha)
-                revno = describe.rsplit("-", 2)[-2]
-                return "%s.a%s.g%s" % (pre_version, revno, sha)
         else:
             return _run_shell_command(
                 "git describe --always").replace('-', '.')