Do not inject git:: to url when git 2.3.0 or above found

[junegunn]

Related: #161
/cc @bruno-

This commit takes advantage of $GIT_TERMINAL_PROMPT variable added in git 2.3.0. So if the git installed on the system is 2.3.0 or above, vim-plug will not use git:: hack to prevent user prompt. However, the downside of the current implementation is that it runs git --version command on startup which should increase the startup time by a little.

[junegunn]

There are two alternatives that come to mind:

1. Defer setting up URI until it's needed. However, this requires some changes in g:plugs structure.
2. Remove git:: part from uri using string substitution before update/install.

[junegunn]

Implemented the second method. (We may need to revert the value of GIT_TERMINAL_PROMPT though.)

[bruno-]

Hi @junegunn,
👍
Much more elegant code than if I had submitted a pull request.
Also, I updated vim-plug locally and tested it a bit and things seem to work fine.

[junegunn]

@bruno- Thanks. I appreciate your feedback!