Inherit stdio for spawned git process

[cy20lin]

Enable user input while deploying, so that git can get authorization info through stdin.

[coveralls]

Coverage remained the same at 90.0% when pulling 0251b1e on cy20lin:cy20lin/master into 056db8e on hexojs:master.

[NoahDragon]

The code LGTM. Could you please elaborate a scenario which can use this change?

[cy20lin]

While I'm using hexo g to deploy blog to github repo through http, it prompts me for my user account and password to continue. However, hexo hangs and doesn't respond to my typing. Since the git process is spawned without inheriting the standard input from parent process (aka hexo), git could never get what it needs to proceed, causing the hangs and could only escape using Ctrl-C.

With this PR, it solves the problem as mentioned.

I'm on Windows MSYS2 mingw64 shell, using nodejs built from MinGW-w64.