mingw: Add platform checking of GoDocBrowser for Cygwin on windows

[rwxguo]

• When using Cygwin/MSYS2 on windows, the :GoDocBrowser cannot popup the browser correctly.
  • Added has('win32unix') for platform compatibility.
• Also resolved vim screen mess up after using :silent with external command
  • refer to :h :silent
  • https://vi.stackexchange.com/questions/2809/silent-makes-my-vim-go-blank

[bhcleek]

Thank you

[rwxguo]

welcome, happy holiday !

[bhcleek]

Thank you for the redraw and that check for cygwin. I'd like to see some changes to the cygwin detection, though.

In addition to the suggested edit I provided, can you add go#util#IsCygwin thusly:

function! IsCygwin()
  return has('win32unix`)
endfunction

Unfortunately, I don't have a windows machine to test with. Can you make the suggested edits and test to make sure they're correct?

I'm not sure vim-go should support mingw in the way that this PR and your others enable, because I think think it's possible that with mingw there's other use cases that would break with the changes here and elsewhere. I'm going to have to give this and your other PRs some thought. For now, can you remove the mingw detection in this PR?

[rwxguo]

@bhcleek Thank you for your guide and instruction. I added cygwin detection go#util#IsCygwin.

The reason using uname to check the platform is inspired by the maven-wrapper project, which is widely used in enterprise level java projects using SpringBoot etc.

Plus, I also leverage the vim built-in feature list has('win32unix') to make it more concise.

I have tested these factors on three major cygwin platforms, the result is for your reference:

platform	has('win32')	has('win64')	has('win32unix')	uname
Cygwin	0	0	1	CYGWIN_...
GitBash (64bit)	0	0	1	MINGW64_...
MSYS2 - mingw32.exe	0	0	1	MINGW32_...
MSYS2 - mingw64.exe	0	0	1	MINGW64_...
MSYS2 - msys2.exe	0	0	1	MSYS_...

[rwxguo]

By the way, I found there is a util function go#util#IsUsingCygwinShell which might not correct, because it uses go#util#IsWin, but based on my above testing result, go#util#IsWin always return 0 on all cygwin-like platform...

 " Checks if using:
 " 1) Windows system,
 " 2) And has cygpath executable,
 " 3) And uses *sh* as 'shell'
function! go#util#IsUsingCygwinShell()
  return go#util#IsWin() && executable('cygpath') && &shell =~ '.*sh.*'
endfunction

The go#util#IsUsingCygwinShell is used in path.vim, for your information

[bhcleek]

Yes, go#util#IsUsingCygwinShell is for a different kind of use case than you've been working on. The last I checked, it is correct for the use case for which it is intended.

[rwxguo]

As CYGWIN uses cygstart instead of start, but MSYS2 and GitBash uses start, I made the cygwin checking as a seprated "elseif", and use rundll32 instead of start rundll32 or cygwin rundll32. (treat cygwin as a separated system, rather than a windows variation)