Account for 'shellslash' being set on Windows

If a user has 'shellslash' set in Windows, then filename related
functionality uses forward slashes instead of backslashes.  This breaks
running of external commands because some external commands in Windows
treat any forward slash as a command line switch.

This change adds s:shellescape(), which wraps Vim's shellescape(), but
ensures 'noshellslash' is set first in Windows command prompt (not
Cygwin or msys) environments.  Having 'noshellslash' set when
shellescape() is called causes the string to be quoted, regardless of
whether there is whitespace or typical special characters, thus avoiding
the interpretation of the string as being command line switches.

Closes jamessan/vim-gnupg#41
Signed-off-by: James McCoy <vega.james@gmail.com>

plugin/gnupg.vim

@@ -214,6 +214,29 @@ highlight default link GPGHighlightUnknownRecipient ErrorMsg
 
 " Section: Functions {{{1
 
+" Function: s:shellescape(s[, special]) {{{2
+"
+" Calls shellescape(), also taking into account 'shellslash'
+" when on Windows and using $COMSPEC as the shell.
+"
+" Returns: shellescaped string
+"
+function s:shellescape(s, ...)
+  let special = a:0 ? a:1 : 0
+  if exists('+shellslash') && &shell == $COMSPEC
+    let ssl = &shellslash
+    set noshellslash
+
+    let escaped = shellescape(a:s, special)
+
+    let &shellslash = ssl
+  else
+    let escaped = shellescape(a:s, special)
+  endif
+
+  return escaped
+endfunction
+
 " Function: s:GPGInit(bufread) {{{2
 "
 " initialize the plugin
@@ -447,7 +470,7 @@ function s:GPGDecrypt(bufread)
 
   " find the recipients of the file
   let cmd = { 'level': 3 }
-  let cmd.args = '--verbose --decrypt --list-only --dry-run --no-use-agent --logger-fd 1 ' . shellescape(filename)
+  let cmd.args = '--verbose --decrypt --list-only --dry-run --no-use-agent --logger-fd 1 ' . s:shellescape(filename)
   let output = s:GPGSystem(cmd)
 
   " Suppress the "N more lines" message when editing a file, not when reading
@@ -530,7 +553,7 @@ function s:GPGDecrypt(bufread)
   " we must redirect stderr (using shell temporarily)
   call s:GPGDebug(1, "decrypting file")
   let cmd = { 'level': 1, 'ex': silent . 'r !' }
-  let cmd.args = '--quiet --decrypt ' . shellescape(filename, 1)
+  let cmd.args = '--quiet --decrypt ' . s:shellescape(filename, 1)
   call s:GPGExecute(cmd)
 
   if (v:shell_error) " message could not be decrypted
@@ -666,7 +689,7 @@ function s:GPGEncrypt()
   let destfile = tempname()
   let cmd = { 'level': 1, 'ex': "'[,']w !" }
   let cmd.args = '--quiet --no-encrypt-to ' . options
-  let cmd.redirect = '>' . shellescape(destfile, 1)
+  let cmd.redirect = '>' . s:shellescape(destfile, 1)
   silent call s:GPGExecute(cmd)
 
   " restore encoding
@@ -1157,7 +1180,7 @@ function s:GPGNameToID(name)
 
   " ask gpg for the id for a name
   let cmd = { 'level': 2 }
-  let cmd.args = '--quiet --with-colons --fixed-list-mode --list-keys ' . shellescape(a:name)
+  let cmd.args = '--quiet --with-colons --fixed-list-mode --list-keys ' . s:shellescape(a:name)
   let output = s:GPGSystem(cmd)
 
   " when called with "--with-colons" gpg encodes its output _ALWAYS_ as UTF-8,
@@ -1322,7 +1345,7 @@ endfunction
 function s:GPGSystem(dict)
   let commandline = s:GPGCommand
   if (!empty(g:GPGHomedir))
-    let commandline .= ' --homedir ' . shellescape(g:GPGHomedir)
+    let commandline .= ' --homedir ' . s:shellescape(g:GPGHomedir)
   endif
   let commandline .= ' ' . a:dict.args
   let commandline .= ' ' . s:stderrredirnull
@@ -1349,7 +1372,7 @@ endfunction
 function s:GPGExecute(dict)
   let commandline = printf('%s%s', a:dict.ex, s:GPGCommand)
   if (!empty(g:GPGHomedir))
-    let commandline .= ' --homedir ' . shellescape(g:GPGHomedir, 1)
+    let commandline .= ' --homedir ' . s:shellescape(g:GPGHomedir, 1)
   endif
   let commandline .= ' ' . a:dict.args
   if (has_key(a:dict, 'redirect'))