[RFC] Minor improvements to vim-patch script.

Generates a file that can directly be used as PR description. Also give some additional hints and changed the file names to `.diff`/`.patch` to comply with Github's naming (e.g. https://github.com/neovim/neovim/pull/1738.diff, https://github.com/neovim/neovim/pull/1738.patch).

Sample output:

```
$ ./scripts/vim-patch.sh 7.4.550
Retrieving Vim sources.
Updating Vim sources in '/home/florian/Projects/vim'.
pulling from https://code.google.com/p/vim
searching for changes
no changes found
✔ Found Vim tag 'v7-4-550'.

Creating Git branch.
✔ Switched to a new branch vim-7.4.550.

Creating empty Neovim commit with correct commit message.
✔ [vim-7.4.550 b6e7239] vim-patch:7.4.550

Creating files.
✔ Saved patch to '/home/florian/Projects/neovim/vim-7.4.550.diff'.
✔ Saved full commit details to '/home/florian/Projects/neovim/vim-7.4.550.patch'.
✔ Saved suggested PR description to '/home/florian/Projects/neovim/vim-7.4.550.pr'.
You can use 'git clean' to remove these files when you're done.

Instructions:

  Proceed to port the patch.
  You might want to try 'patch -p1 < vim-7.4.550.diff' first.

  Stage your changes ('git add ...') and use 'git commit --amend' to commit.

  Push your changes with 'git push origin vim-7.4.550' and create a
  pull request called '[RFC] vim-patch:7.4.550'. You might want 
  to use the text in 'vim-7.4.550.pr' as the description of this pull request.

  See https://github.com/neovim/neovim/wiki/Merging-patches-from-upstream-vim
  for more information.
$ ./scripts/vim-patch.sh 7.4.550
Retrieving Vim sources.
Updating Vim sources in '/home/florian/Projects/vim'.
pulling from https://code.google.com/p/vim
searching for changes
no changes found
✔ Found Vim tag 'v7-4-550'.

Creating Git branch.
✘ fatal: A branch named vim-7.4.550 already exists.
```