Fix delay with flicking through files or commits when git diff is very slow

One reason why git diff can be very slow is when "diff.algorithm = histogram" is
being used. In this case, showing a very long single-file diff can take seconds
to load, and you'll see the "loading..." message in the main view until we got
the first lines of the diff to show. There's nothing really we can do about this
delay; however, when switching to another, shorter file (or commit) while the
"loading..." message is still showing, this switch should be instantaneous. And
it was before 0.54.0, but we broke this in 0.54.0 with 8d7740a5ac (#4782); now
users have to wait for the slow git diff command to output more text before the
switch occurs.

To fix this, don't block waiting for the process to terminate if we just stopped
it.

pkg/tasks/tasks.go

@@ -291,11 +291,15 @@ func (self *ViewBufferManager) NewCmdTask(start func() (*exec.Cmd, io.Reader), p
 
 			refreshViewIfStale()
 
-			if err := cmd.Wait(); err != nil {
-				select {
-				case <-opts.Stop:
-					// it's fine if we've killed this program ourselves
-				default:
+			select {
+			case <-opts.Stop:
+				// If we stopped the task, don't block waiting for it; this could cause a delay if
+				// the process takes a while until it actually terminates. We still want to call
+				// Wait to reclaim any resources, but do it on a background goroutine, and ignore
+				// any errors.
+				go func() { _ = cmd.Wait() }()
+			default:
+				if err := cmd.Wait(); err != nil {
 					self.Log.Errorf("Unexpected error when running cmd task: %v; Failed command: %v %v", err, cmd.Path, cmd.Args)
 				}
 			}