libgit2-sys: use src instead .git as vendored indicator

When someone vendored `libgit2-sys` he may exclude `.git` folder.

When such things happened an attempt to build it may lead to error like:
`fatal: not a git repository (or any of the parent directories): .git`.

The only way to fix it is using system's `libgit2` which may lead to
`invalid version 0 on git_proxy_options; class=Invalid (3)` at some
cases.

Backported as: rust-lang/git2-rs#801

vendor/libgit2-sys/build.rs

@@ -20,7 +20,7 @@ fn main() {
         }
     }
 
-    if !Path::new("libgit2/.git").exists() {
+    if !Path::new("libgit2/src").exists() {
         let _ = Command::new("git")
             .args(&["submodule", "update", "--init", "libgit2"])
             .status();