Support tilde expansion when completing files

- Expand tilde alone to $HOME (same as vim's built in path completion)
- Support completion of paths relative to ~/ (taken from PR mhinz#152)

This is getting quite messy now, but that's ok, we have some tests!
Refactoring can come later, maybe when supporting hidden files/dirs.

plugin/grepper.vim

@@ -235,8 +235,14 @@ endfunction
 " grepper#complete_files() {{{2
 function! grepper#complete_files(lead, _line, _pos)
   let [head, path] = s:extract_path(a:lead)
+  " tilde expansion to $HOME
+  if path ==# '~'
+    return [head . $HOME]
+  " handle paths in $HOME (~/foo)
+  elseif path[0:1] ==# '~/'
+    return map(split(globpath($HOME, path[2:].'*'), '\n'), 'head . "~" . v:val['.len($HOME).':] . (isdirectory(v:val) ? s:slash : "")')
   " handle relative paths
-  if empty(path) || (path =~ '\s$') || (path =~ '^\s*\w\+')
+  elseif empty(path) || (path =~ '\s$') || (path =~ '^\s*\w\+')
     return map(
           \ map(split(globpath('.'.s:slash, path.'*'), '\n'), "substitute(v:val, '^\\s*.'.s:slash, '', '')"),
           \ 'head . v:val . (isdirectory(v:val) ? s:slash : "")'

test/feature/completion.vader

@@ -37,6 +37,15 @@ Execute (command: flags, -tool options):
   Assert len(grepper#complete('', 'Grepper -tool ', v:null)) > 1
   AssertEqual count(grepper#complete('', 'Grepper -tool ', v:null), 'grep '), 1
 
+Execute (prompt: path, tilde expansion):
+  AssertEqual grepper#complete_files('x ~', v:null, v:null), ['x '.$HOME]
+
+Execute (prompt: path, relative to $HOME):
+  let home = $HOME
+  let $HOME = getcwd()
+  AssertEqual grepper#complete_files('x ~/f', v:null, v:null), ['x ~/foo/']
+  let $HOME = home
+
 Execute (prompt: relative path, empty string):
   AssertEqual grepper#complete_files('x ', v:null, v:null), ['x foo/']