Parse story ID from branch name correctly

There was an edge case with feature branches having description in the
branch name starting with a digit. Then the regexp to parse story id would
fail.

This patch makes the thing a bit more explicit and more robust, handling
the edge case correctly.

Signed-off-by: Ondrej Kupka <ondra.cap@gmail.com>
Finished feature 54595696/feature-finish-throws-404.