[fzf#vim#with_preview] Support WSL bash (#1026)

* [fzf#vim#with_preview] support wsl bash

Close #988
Close #1009

* [fzf#vim#with_preview] use abs path for bin/

WSL has issues with relative filepaths.

autoload/fzf/vim.vim

@@ -30,7 +30,7 @@ set cpo&vim
 
 let s:is_win = has('win32') || has('win64')
 let s:layout_keys = ['window', 'up', 'down', 'left', 'right']
-let s:bin_dir = expand('<sfile>:h:h:h').'/bin/'
+let s:bin_dir = expand('<sfile>:p:h:h:h').'/bin/'
 let s:bin = {
 \ 'preview': s:bin_dir.'preview.sh',
 \ 'tags':    s:bin_dir.'tags.pl' }
@@ -41,7 +41,6 @@ if s:is_win
   else
     let s:bin.preview = fnamemodify(s:bin.preview, ':8')
   endif
-  let s:bin.preview = 'bash '.escape(s:bin.preview, '\')
 endif
 
 let s:wide = 120
@@ -76,6 +75,11 @@ endfunction
 
 " [[options to wrap], [preview window expression], [toggle-preview keys...]]
 function! fzf#vim#with_preview(...)
+  let bash_path = exepath('bash')
+  if empty(bash_path)
+    throw 'bash is not in PATH'
+  endif
+  let is_wsl_bash = bash_path =~? 'Windows[/\\]system32[/\\]bash.exe$'
   " Default options
   let options = {}
   let window = ''
@@ -104,7 +108,14 @@ function! fzf#vim#with_preview(...)
   if len(window)
     let preview += ['--preview-window', window]
   endif
-  let preview += ['--preview', (s:is_win ? s:bin.preview : fzf#shellescape(s:bin.preview)).' '.placeholder]
+  if s:is_win
+    let preview_cmd = 'bash '.(is_wsl_bash
+    \ ? substitute(substitute(s:bin.preview, '^\([A-Z]\):', '/mnt/\L\1', ''), '\', '/', 'g')
+    \ : escape(s:bin.preview, '\'))
+  else
+    let preview_cmd = fzf#shellescape(s:bin.preview)
+  endif
+  let preview += ['--preview', preview_cmd.' '.placeholder]
 
   if len(args)
     call extend(preview, ['--bind', join(map(args, 'v:val.":toggle-preview"'), ',')])