fix(git): handle non-unix stderr redirection

[scottmckendry]

This PR is specifically for the Windows crowd.

I tried running this plugin on my Windows machine and was getting a blank ASCII. After some head-scratching, I was able to determine this was due to the Unix-style stderr redirection on a couple of the functions in the git.lua file.

I've fixed this by adding two new local variables and using the built-in vim.fn.has API to determine the correct output.

[juansalvatore]

I just tried it out, thanks for the improvement!

[noahlias]

The vim.fn.has() return 0|1 not boolean,
this works on windows but not on unix
it will generate a NUL file always

It should be modifed.

local is_windows = vim.fn.has("win32") ==1