Add g:plug_disable_credential_helper (default: 1)

This can be set to '0' not to disable git credential helper.

    let g:plug_disable_credential_helper = 0

See #1026
Close #1047

plug.vim

@@ -1075,12 +1075,16 @@ function! s:hash_match(a, b)
   return stridx(a:a, a:b) == 0 || stridx(a:b, a:a) == 0
 endfunction
 
+function! s:disable_credential_helper()
+  return s:git_version_requirement(2) && get(g:, 'plug_disable_credential_helper', 1)
+endfunction
+
 function! s:checkout(spec)
   let sha = a:spec.commit
   let output = s:git_revision(a:spec.dir)
   let error = 0
   if !empty(output) && !s:hash_match(sha, s:lines(output)[0])
-    let credential_helper = s:git_version_requirement(2) ? '-c credential.helper= ' : ''
+    let credential_helper = s:disable_credential_helper() ? '-c credential.helper= ' : ''
     let output = s:system(
           \ 'git '.credential_helper.'fetch --depth 999999 && git checkout '.plug#shellescape(sha).' --', a:spec.dir)
     let error = v:shell_error
@@ -1589,7 +1593,7 @@ while 1 " Without TCO, Vim stack is bound to explode
     let [error, _] = s:git_validate(spec, 0)
     if empty(error)
       if pull
-        let cmd = s:git_version_requirement(2) ? ['git', '-c', 'credential.helper=', 'fetch'] : ['git', 'fetch']
+        let cmd = s:disable_credential_helper() ? ['git', '-c', 'credential.helper=', 'fetch'] : ['git', 'fetch']
         if has_tag && !empty(globpath(spec.dir, '.git/shallow'))
           call extend(cmd, ['--depth', '99999999'])
         endif