Handle local file:/// paths on Windows

Windows can't handle a path like `/c:/foo`; when turning file:///
URIs into local paths, we must strip the leading slash.
Export git_buf_text_is_binary and git_buf_text_contains_nul.

So that users don’t need to implement binary detection themselves.
Make sure \n is at the end of config file before a new section is written
Add unit test to test add section without lf at EOF
Merge pull request #2467 from ethomson/win_local_clone

Handle local file:/// paths on Windows
clone: should_clone? Of course we should clone. That's not the question
ssh: Fix unused warning
Merge pull request #2476 from linquize/config-lf-eof

When adding new config section, handle config file not ending with LF
Just put it all in buffer.
Merge pull request #2475 from libgit2/expose-buffer-binary-detection

Export git_buf_text_is_binary and git_buf_text_contains_nul.