add completions for source and document the move away from '.'

Closes: #310
use docdir from configure in setting __fish_help_dir
Closes #1065
set docdir in Xcode build
fixes part of #1070
Link with $CXXFLAGS; disable GCC-specific autoconf check (squash fixes).
update URL for nightly builds
Make type -a print all executables in PATH
clobber the prompt when it exceeds the screen width, not when it equals it

Closes #1069
Fix broken links in tutorial
Implement file autocompletion for git add and git checkout.

Fixes bpinto/oh-my-fish#59.