Fix KeyboardInterrupt on git-start

```
⌘ git start
Retrieving next available story from Pivotal Tracker...
Story: Keep a mongoose
Enter branch name: ^C
Aborted story branch.
Traceback (most recent call last):
  File "<string>", line 16, in <module>
  File "/private/tmp/continuity-1DRzO2/continuity-0.9.6/build/continuity/out00-PYZ.pyz/continuity.cli", line 134, in main
  File "/private/tmp/continuity-1DRzO2/continuity-0.9.6/build/continuity/out00-PYZ.pyz/continuity.cli.commons", line 90, in __call__
  File "/private/tmp/continuity-1DRzO2/continuity-0.9.6/build/continuity/out00-PYZ.pyz/continuity.cli.pt", line 266, in exit
  File "/private/tmp/continuity-1DRzO2/continuity-0.9.6/build/continuity/out00-PYZ.pyz/async", line 21, in thread_interrupt_handler
KeyboardInterrupt
```