gitk: Avoid handling the Return key twice in Add Branch

gitk: Avoid handling the Return key twice in Add Branch

This reverts commit 63767d5fb8fe236d8fdeba44297ac925701b27a0.

A similar change was made as part of commit 76f15947af7, that added
bindings to all dialogs, and this duplication causes mkbrgo to be
called twice, the second time after the window has been destroyed.
As a result, an error window appears when the code tries to access
widgets.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>