`fork-point --override-to...`: use short hash consistently in confirmation message

[PawelLipski]

Currently, both short and long hash are used:

Fork point for feature/foo is overridden to master (commit ce02c372e669).
This applies as long as feature/foo is a descendant of commit ce02c372e669547751be0bf4682adf661dc9c3cd.

Let's use the short hash in the latter sentence as well.