Kill background fetch when it requests a passphrase

Previously we would enter a newline at the password prompt, which would cause
the fetch to fail. The problem with this was that if you have many remotes, the
fetch would sometimes hang for some reason; I don't totally understand how that
happened, but I guess the many ssh processes requesting passwords would somehow
interfere with each other. Avoid this by simply killing the git fetch process
the moment it requests the first password.

pkg/commands/oscommands/cmd_obj_runner.go

@@ -294,14 +294,10 @@ const (
 	Token
 )
 
-// Whenever we're asked for a password we just enter a newline, which will
-// eventually cause the command to fail.
+// Whenever we're asked for a password we return a nil channel to tell the
+// caller to kill the process.
 var failPromptFn = func(CredentialType) <-chan string {
-	ch := make(chan string)
-	go func() {
-		ch <- "\n"
-	}()
-	return ch
+	return nil
 }
 
 func (self *cmdObjRunner) runWithCredentialHandling(cmdObj *CmdObj) error {
@@ -360,16 +356,26 @@ func (self *cmdObjRunner) processOutput(
 		askFor, ok := checkForCredentialRequest(newBytes)
 		if ok {
 			responseChan := promptUserForCredential(askFor)
-			if task != nil {
-				task.Pause()
-			}
-			toInput := <-responseChan
-			if task != nil {
-				task.Continue()
-			}
-			// If the return data is empty we don't write anything to stdin
-			if toInput != "" {
-				_, _ = writer.Write([]byte(toInput))
+			if responseChan == nil {
+				// Returning a nil channel means we should kill the process.
+				// Note that we don't break the loop after this, because we
+				// still need to drain the output, otherwise the Wait() call
+				// later might block.
+				if err := Kill(cmdObj.GetCmd()); err != nil {
+					self.log.Error(err)
+				}
+			} else {
+				if task != nil {
+					task.Pause()
+				}
+				toInput := <-responseChan
+				if task != nil {
+					task.Continue()
+				}
+				// If the return data is empty we don't write anything to stdin
+				if toInput != "" {
+					_, _ = writer.Write([]byte(toInput))
+				}
 			}
 		}
 	}