fix ignore BufEnter and BufLeave events when create popup window

[Alex-duzhichao]

After popup window has been created, git-messenager uses
noautocmd wincmd p to jump back to the current buffer, which
ignores BufEnter and BufLeave events. Some other plugins depends
on event BufEnter/BufLeave to take effect normally. So git-messenager
should use wincmd p to preserve events.

See issue #65 for details.

[rhysd]

Did you confirm that this causes no issue? I concern that the following auto commands are unexpectedly triggered by this change.

git-messenger.vim/autoload/gitmessenger/popup.vim

Line 211 in 2e67899

execute 'autocmd BufWipeout,BufLeave <buffer> call getbufvar(' . popup_bufnr . ', "__gitmessenger_popup").close()'

git-messenger.vim/autoload/gitmessenger.vim

Line 70 in 2e67899

execute 'autocmd BufEnter,WinEnter * call <SID>on_buf_enter(' . opener_bufnr . ')'

git-messenger.vim/autoload/gitmessenger.vim

Line 65 in 2e67899

autocmd CursorMoved,CursorMovedI,InsertEnter <buffer> call <SID>on_cursor_moved()