path.c: Learn invocation_path_tail().

Required for vim patch 276 as an alternative to
`get_isolated_shell_name()`.
Doxygen/comment style for make_filter_cmd().
vim-patch:7.4.276

Problem:    The fish shell is not supported.
Solution:   Use begin/end instead of () for fish. (Andy Russell)

https://code.google.com/p/vim/source/detail?r=a6b59ee633a355095e6473ec5e2a7d9088bfb853