prompt: fix cursor position for prompts longer than one character

This can be reproduced by opening the grep view from within Tig while
the main view is loading in the background. The prompt position was
assuming that the prompt has a size of one.
blame: parse author email to support author toggling
Use email instead of name as the key in the author ident cache

This ensures that toggling will work correctling when there are two
authors with the same name.