From: Mikhail Terekhov <termim@gmail.com>

In proc _is_git check that supplied path is a valid work tree path.
This allows the choose_repository::pick dialog to accept path to a
work tree directory.

Signed-off-by: Mikhail Terekhov <termim@gmail.com>

Submitted-As:  Message-ID: pull.644.v2.git.1591418019023.gitgitgadget@gmail.com
In-Reply-To:  Message-ID: pull.644.git.1590507201474.gitgitgadget@gmail.com