gitk: Fix binding for <Return> in sha1 entry field

gitk: Fix binding for <Return> in sha1 entry field

This adds a break so that gitk doesn't go and execute the global
binding for <Return> (i.e. find next) when the user presses the
return key in the sha1 entry field to indicate that gitk should
jump to the commit identified by what they just put into the
sha1 field.

Signed-off-by: Paul Mackerras <paulus@samba.org>