Added package diff-so-fancy and make diff use it by default for its
diffs by setting it as its diff-pager.
Added sanity function dsf which invokes diff-so-fancy if it exists or
degrades gracefully if it does not. Use this function for all git diff
needs.
Pre-fill the pager search term in git to enable jumping between changed
files with n/N.
Added general usage information. Made sure gitignore api is never called
without any arguments.
Made script fzf aware -- if it finds fzf it can be invoked without
arguments, if it does not it will display usage information instead.
