[RFC] vim-patch:7.4.276

Neovim already has two pull requests for this patch: #841 and #773 by @euclio, the original author of the vanilla patch. It adds support for the fish shell. Without patch 276, fish users can only use plugins like syntastic if they have, in their .vimrc, `set shell=sh` or something similar.

I wasn't quite sure what to do with this for a while.  At first, I branched off of his patch and appended commits until it worked: https://github.com/splinterofchaos/neovim/tree/fish-patch. I found that travis kept failing it due to a memory leak introduced by [276](https://code.google.com/p/vim/source/detail?spec=svna6b59ee633a355095e6473ec5e2a7d9088bfb853&r=a6b59ee633a355095e6473ec5e2a7d9088bfb853) and solved by [297](https://code.google.com/p/vim/source/detail?r=81f5a056b2a582c8109da10cc538dc16a326a34d). 

So it seemed more logical to avoid the allocation altogether thus making 297 moot. Since neovim has `path_tail()`, I piggy-backed off that than implement `get_isolated_shell_name()`. The new `path_tail_save()` does the exact same thing, though I wondered if others might find it weird that it works for program invocations as well.
I think we can avoid this cast because there is no point in calling to `get_past_head`. `get_past_head` just skips path separators in the beginning of the path, but we can go through them in the loop below. And in all cases the behavior of the `invocation_path_tail` seems to be the same. So I suggest to change this line to
```c
const char_u *tail = invocation;
```