kubectx rename check if old_name is a valid ctx (#139)
kubectx rename check if old_name is a valid ctx (#139)

Without this safeguard, when user runs `kubectx NEW_NAME=OLD_NAME` where
NEW_NAME is an existing context but OLD_NAME isn't, we end up deleting NEW_NAME
and not doing any renames (because OLD_NAME is not found).

Fixes #136.

Signed-off-by: Ahmet Alp Balkan <c786c8527fd6be7431bb94ca48b6f76cc06787cc@google.com>
ahmetb/kubectx,ahmetb/kubectx