Panic if libgit2 is not thread-aware

Go calling C is inherently multi-threaded. If libgit2 cannot handle
threading, then we're going to crash at some random point. Crash right
at the start so we know what's happening.

git.go

@@ -124,6 +124,15 @@ func init() {
 
 	C.git_libgit2_init()
 
+	// Due to the multithreaded nature of Go and its interaction with
+	// calling C functions, we cannot work with a library that was not built
+	// with multi-threading support. The most likely outcome is a segfault
+	// or panic at an incomprehensible time, so let's make it easy by
+	// panicking right here.
+	if Features()&FeatureThreads == 0 {
+		panic("libgit2 was not built with threading support")
+	}
+
 	// This is not something we should be doing, as we may be
 	// stomping all over someone else's setup. The user should do
 	// this themselves or use some binding/wrapper which does it