Make gf not prompt if file exists in normal state

Originally reported by: **nixmaniack (Bitbucket: [nixmaniack](https://bitbucket.org/nixmaniack), GitHub: [nixmaniack](https://github.com/nixmaniack))**

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### System Info

* OS: darwin
* Emacs: 24.5.1
* Evil: changeset: 1733:b9a6f0c1ab9f (this is from hg log | head -1)

### Description

`gf` shouldn't prompt if file already exits and visitable. The normal behaviour of vim is to visit that file directly.

### Reproduction guide

Start Emacs using `make terminal` in evil source.

On a valid file under point(e.g. ~/.emacs.d/init.el), in normal state, press `gf`

**Observed behaviour:**
Opens up prompt where you have to hit `RET` to visit the file.

**Expected behaviour:**
You shouldn't need to hit return in case file is present and readable.

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- Bitbucket: https://bitbucket.org/lyro/evil/issue/614

Start the Evil source with `make terminal`, place point on a valid readable file such as `~/.emacs.d/init.el`, and press `gf` in normal state to reproduce the prompt. Trace the `gf` entry point and confirm that an existing visitable file opens directly without requiring RET, while preserving prompting for other cases.

Written by the indexing model from the issue text.