fix: work around breakage caused by rg v13.0.0

v13.0.0 broke a number of tools in Neovim, but not in Vim, as described
here:

    BurntSushi/ripgrep#1892

My reading of that is that changes to Neovim will/may be required,
because the `rg` author is right that rg's handling is "technically
correct" and I don't think he's going to change it. Luckily, we don't
have to take sides in that argument because there is a straightforward
enough workaround of always providing a path to search (ie. ".", only if
one isn't already provided by the user).

<tangential-rant>

As I lamented here:

    https://twitter.com/wincent/status/1413536091425198082

installing and building docvim on this machine (in order to include the
update to the "HISTORY" section in the docs) is a frickin' ordeal. It
takes:

1. 5.53s to clone the repo.
2. 3m56s to install GHC.
3. 3m24s to build the project.
4. 1.64s to run the built tool.

Which seems massively out-of-proportion for something that clocks in at
1,380 lines of Haskell according to `sloccount`. On the upside, this is
one of the few tasks I can throw at this Linux box that actually taxes
the machine. 😆

Steps:

    git clone git@git.wincent.com:public/docvim.git

    cd docvim

    bin/docvim \
      -c ~/.vim/pack/bundle/opt/ferret \
      ~/.vim/pack/bundle/opt/ferret/README.md \
      ~/.vim/pack/bundle/opt/ferret/doc/ferret.txt

    # That did't work; need to build first, then retry:
    bin/build

    bin/docvim \
      -c ~/.vim/pack/bundle/opt/ferret \
      ~/.vim/pack/bundle/opt/ferret/README.md \
      ~/.vim/pack/bundle/opt/ferret/doc/ferret.txt

Closes: #78

README.md

@@ -712,6 +712,7 @@ This list produced with:
 - Use `:normal!` instead of <strong>`:normal`</strong> to avoid running custom mappings (patch from Yoni Weill, https://github.com/wincent/ferret/pull/67).
 - Append a trailing slash when autocompleting a directory name (https://github.com/wincent/ferret/issues/69).
 - Fixed failure to detect pre-existing mapping to <strong>[`<Plug>(FerretLack)`](#user-content-plugferretlack)</strong>.
+- Worked around breakage caused by `rg` v13.0.0 (https://github.com/wincent/ferret/issues/78).
 
 ### 5.0 (8 June 2019)<a name="ferret-50-8-june-2019" href="#user-content-ferret-50-8-june-2019"></a>
 

autoload/ferret/private/nvim.vim

@@ -12,8 +12,30 @@ endfunction
 function! ferret#private#nvim#search(command, ack, bang) abort
   call ferret#private#nvim#cancel()
   call ferret#private#autocmd('FerretAsyncStart')
-  let l:command_and_args=extend(split(ferret#private#executable()), a:command)
-  let l:job=jobstart(l:command_and_args, {
+  let l:executable=split(ferret#private#executable())
+  let l:default_search_paths=[]
+  if l:executable[0] ==# 'rg'
+    " Hack for breakage caused by rg v13.0.0;
+    " see: https://github.com/wincent/ferret/issues/78
+    let l:seen_search_term=0
+    let l:seen_search_path=0
+    for l:arg in a:command
+      if !ferret#private#option(l:arg)
+        if !l:seen_search_term
+          let l:seen_search_term=1
+        else
+          let l:seen_search_path=1
+          break
+        end
+      end
+    endfor
+    if !l:seen_search_path
+      call extend(l:default_search_paths, ['.'])
+    endif
+  endif
+  call extend(l:executable, a:command)
+  call extend(l:executable, l:default_search_paths)
+  let l:job=jobstart(l:executable, {
         \   'on_stderr': 'ferret#private#nvim#err_cb',
         \   'on_stdout': 'ferret#private#nvim#out_cb',
         \   'on_exit': 'ferret#private#nvim#close_cb'

doc/ferret.txt

@@ -833,6 +833,8 @@ main (not yet released) ~
 - Append a trailing slash when autocompleting a directory name
   (https://github.com/wincent/ferret/issues/69).
 - Fixed failure to detect pre-existing mapping to |<Plug>(FerretLack)|.
+- Worked around breakage caused by `rg` v13.0.0
+  (https://github.com/wincent/ferret/issues/78).
 
 5.0 (8 June 2019) ~
 

plugin/ferret.vim

@@ -397,6 +397,8 @@
 " - Append a trailing slash when autocompleting a directory name
 "   (https://github.com/wincent/ferret/issues/69).
 " - Fixed failure to detect pre-existing mapping to |<Plug>(FerretLack)|.
+" - Worked around breakage caused by `rg` v13.0.0
+"   (https://github.com/wincent/ferret/issues/78).
 "
 " ## 5.0 (8 June 2019)
 "