Use threads only for git processes on windows

On Ubuntu, threads can cause seg faults, see:
HaxeFoundation/neko#281

src/haxelib/client/Vcs.hx

@@ -158,32 +158,46 @@ class Vcs implements IVcs {
         		code: -1,
         		out: Std.string(e)
         	}
-        }
-		var streamsLock = new sys.thread.Lock();
-		function readFrom(stream:haxe.io.Input, to: {value: String}) {
-			to.value = stream.readAll().toString();
-			streamsLock.release();
-		}
+		};
+		var ret = if (Sys.systemName() == "Windows") {
+			var streamsLock = new sys.thread.Lock();
+			function readFrom(stream:haxe.io.Input, to: {value: String}) {
+				to.value = stream.readAll().toString();
+				streamsLock.release();
+			}
 
-		var out = {value: ""};
-		var err = {value: ""};
-		Thread.create(readFrom.bind(p.stdout, out));
-		Thread.create(readFrom.bind(p.stderr, err));
+			var out = {value: ""};
+			var err = {value: ""};
+			Thread.create(readFrom.bind(p.stdout, out));
+			Thread.create(readFrom.bind(p.stderr, err));
 
-		var code = p.exitCode();
-		for (_ in 0...2) {
-			// wait until we finish reading from both streams
-			streamsLock.wait();
-		}
+			var code = p.exitCode();
+			for (_ in 0...2) {
+				// wait until we finish reading from both streams
+				streamsLock.wait();
+			}
 
-		if (settings.debug && out.value != "")
-			Sys.println(out.value);
-		if (settings.debug && err.value != "")
-			Sys.stderr().writeString(err.value);
+			if (settings.debug && out.value != "")
+				Sys.println(out.value);
+			if (settings.debug && err.value != "")
+				Sys.stderr().writeString(err.value);
 
-		var ret = {
-			code: code,
-			out: code == 0 ? out.value : err.value
+			{
+				code: code,
+				out: code == 0 ? out.value : err.value
+			};
+		} else {
+			var out = p.stdout.readAll().toString();
+			var err = p.stderr.readAll().toString();
+			if (settings.debug && out != "")
+				Sys.println(out);
+			if (settings.debug && err != "")
+				Sys.stderr().writeString(err);
+			var code = p.exitCode();
+			{
+				code: code,
+				out: code == 0 ? out : err
+			};
 		};
         p.close();
         return ret;