Add option for preamble before shdo scripts

I want all of my scripts to start in the current dirvish working
directory, so I use the example from the doc:
  let g:dirvish_shdo_before = 'cd {}'

Other users may tend to create scripts with arguments, want their
commands to be inside a function, or output some diagnostic data.

Using the same escaping that is applied on lines since that seems like
the safest thing to do.

autoload/dirvish.vim

@@ -119,6 +119,11 @@ function! dirvish#shdo(paths, cmd) abort
     let f = !jagged && 2==exists(':lcd') ? fnamemodify(f, ':t') : lines[i]
     let lines[i] = substitute(cmd, '\V{}', escape(shellescape(f),'&\'), 'g')
   endfor
+
+  if !empty(g:dirvish_shdo_before)
+    let lines = [substitute(g:dirvish_shdo_before, '\V{}', escape(shellescape(head),'&\'), 'g'), ''] + lines
+  endif
+
   execute 'silent split' tmpfile '|' (2==exists(':lcd')?('lcd '.dir):'')
   setlocal bufhidden=wipe
   silent keepmarks keepjumps call setline(1, lines)

doc/dirvish.txt

@@ -165,6 +165,20 @@ can be overridden by handling that event. Example: >
         \ gh :silent keeppatterns g@\v/\.[^\/]+/?$@d _<cr>:setl cole=3<cr>
     augroup END
 <
+g:dirvish_shdo_before = ''                          *g:dirvish_shdo_before*
+    Inserts this text at the beginning of shell scripts generated with
+    |:Shdo|. The current dirvish directory is inserted wherever {} appears in
+    |g:dirvish_shdo_before|.
+    To ensure scripts always cd to the current dirvish directory, you could
+    add this to your vimrc: >
+
+    if has('win32')
+      " work across hard drives
+      let g:dirvish_shdo_before = 'pushd {}'
+    else
+      let g:dirvish_shdo_before = 'cd {}'
+    endif
+<
 
 ==============================================================================
 FAQ                                                              *dirvish-faq*

plugin/dirvish.vim

@@ -3,6 +3,8 @@ if exists('g:loaded_dirvish') || &cp || v:version < 700 || &cpo =~# 'C'
 endif
 let g:loaded_dirvish = 1
 
+let g:dirvish_shdo_before = get(g:, 'dirvish_shdo_before', '')
+
 command! -bar -nargs=? -complete=dir Dirvish call dirvish#open(<q-args>)
 command! -nargs=* -complete=file -range -bang Shdo call dirvish#shdo(<bang>0 ? argv() : getline(<line1>, <line2>), <q-args>)