Fix pid functions with cygwin shells

The actually problem is that vim automatically set &shellquote='"' when
&shell is some kind of posix-like shells. When &shellquote is '"',
writing double quotes in the command passed to system() would cause the
command to be misinterpreted, and producing the weird error as reported
in #108.  The solution is to detect whether shellquote is set to double
quote, and alter the sent command accordingly.

Note that this issue would not affect default windows cmd shell, thus
the command should only be altered when *sh shells are used.

Fixes #108.

autoload/dispatch.vim

@@ -581,7 +581,11 @@ function! s:running(pid) abort
   if !a:pid
     return 0
   elseif has('win32')
-    return system('tasklist /fi "pid eq '.a:pid.'"') =~# '==='
+    let tasklist_cmd = 'tasklist /fi "pid eq '.a:pid.'"'
+    if &shellxquote ==# '"'
+      let tasklist_cmd = substitute(tasklist_cmd, '"', "'", "g")
+    endif
+    return system(tasklist_cmd) =~# '==='
   else
     call system('kill -0 '.a:pid)
     return !v:shell_error

autoload/dispatch/windows.vim

@@ -71,9 +71,14 @@ function! dispatch#windows#start(request) abort
 endfunction
 
 function! dispatch#windows#activate(pid) abort
-  if system('tasklist /fi "pid eq '.a:pid.'"') !~# '==='
+  let tasklist_cmd = 'tasklist /fi "pid eq '.a:pid.'"'
+  if &shellxquote ==# '"'
+    let tasklist_cmd = substitute(tasklist_cmd, '"', "'", "g")
+  endif
+  if system(tasklist_cmd) !~# '==='
     return 0
   endif
+
   if !exists('s:activator')
     let s:activator = tempname().'.vbs'
     call writefile(['WScript.CreateObject("WScript.Shell").AppActivate(WScript.Arguments(0))'], s:activator)