Fix #139 - Use git fetch+merge instead of pull

test/regressions.vader

@@ -72,3 +72,80 @@ Execute (#131 Syntax error):
   silent PlugInstall
   redir END
   Assert out =~ 'Invalid plug directory: /no-permission'
+
+**********************************************************************
+Execute (#139-1 Using new remote branch):
+  " Make sure to remove the clone
+  call plug#begin('$TMPDIR/plugged')
+  call plug#end()
+  PlugClean!
+
+  " Install master branch
+  call plug#begin('$TMPDIR/plugged')
+  Plug expand('file:///$TMPDIR/new-branch')
+  call plug#end()
+  PlugUpdate
+
+  unlet! g:foo g:bar g:baz
+  call plug#load('new-branch')
+  Assert exists('g:foo'),  'g:foo should be found'
+  Assert !exists('g:bar'), 'g:bar should not be found'
+  Assert !exists('g:baz'), 'g:baz should not be found'
+
+  " Create a new branch on origin
+  call system('cd $TMPDIR/new-branch && git checkout -b new &&'
+      \. 'echo "let g:bar = 1" > plugin/bar.vim && git add plugin/bar.vim &&'
+      \. 'git commit -m second')
+
+  " We're setting up two plugins so that parallel installer is used
+  call plug#begin('$TMPDIR/plugged')
+  Plug 'junegunn/seoul256.vim'
+  Plug expand('file:///$TMPDIR/new-branch'), 'new'
+  call plug#end()
+  PlugUpdate
+  silent %y
+  Log @"
+  Assert @" !~? 'error', 'Should be able to use new remote branch: ' . @"
+
+  unlet! g:foo g:bar g:baz
+  call plug#load('new-branch')
+  Assert exists('g:foo'),  'g:foo should be found'
+  Assert exists('g:bar'),  'g:bar should be found'
+  Assert !exists('g:baz'), 'g:baz should not be found'
+
+  call PlugStatusSorted()
+
+Expect:
+  - new-branch: OK
+  - seoul256.vim: OK
+  Finished. 0 error(s).
+  [==]
+
+Execute (#139-2 Using yet another new remote branch):
+  " Create another branch on origin
+  call system('cd $TMPDIR/new-branch && git checkout master &&'
+      \. 'git checkout -b brand-new &&'
+      \. 'echo "let g:baz = 1" > plugin/baz.vim && git add plugin/baz.vim &&'
+      \. 'git commit -m thirf')
+
+  " Test Vim installer here
+  call plug#begin('$TMPDIR/plugged')
+  Plug expand('file:///$TMPDIR/new-branch'), 'brand-new'
+  call plug#end()
+  PlugUpdate
+  silent %y
+  Log @"
+  Assert @" !~? 'error', 'Should be able to use new remote branch: ' . @"
+
+  unlet! g:foo g:bar g:baz
+  call plug#load('new-branch')
+  Assert exists('g:foo'),  'g:foo should be found'
+  Assert !exists('g:bar'), 'g:bar should not be found'
+  Assert exists('g:baz'),  'g:baz should be found'
+
+  call PlugStatusSorted()
+
+Expect:
+  - new-branch: OK
+  Finished. 0 error(s).
+  [=]