mingw: Modify GoDoc split window key binding for GitBash compatibility

[rwxguo]

The following keybinding resolved leading escape as a prefix (e.g. arrow keys), But on GitBash, as there is no leading escape issue when press arrow keys, when press Esc on the :GoDoc scratch window, the window will not be closed.

nnoremap <buffer> <silent> <Esc>[ <Esc[

Thus, I made a GitBash platform checking to skip the key binding.