Improve CPU usage when flicking through large diffs

The previous commit already fixed the user-visible lag, but there's still a
problem with multiple background git processes consuming resources calculating
diffs that we are never going to show. Improve this by terminating those
processes (by sending them a TERM signal).

Unfortunately this is only possible on Linux and Mac, so Windows users will have
to live with the higher CPU usage. The recommended workaround is to not use
"diff.algorithm = histogram".

pkg/commands/oscommands/os_default_platform.go

@@ -5,8 +5,10 @@ package oscommands
 
 import (
 	"os"
+	"os/exec"
 	"runtime"
 	"strings"
+	"syscall"
 )
 
 func GetPlatform() *Platform {
@@ -38,3 +40,11 @@ func getUserShell() string {
 func (c *OSCommand) UpdateWindowTitle() error {
 	return nil
 }
+
+func TerminateProcessGracefully(cmd *exec.Cmd) error {
+	if cmd.Process == nil {
+		return nil
+	}
+
+	return cmd.Process.Signal(syscall.SIGTERM)
+}

pkg/commands/oscommands/os_windows.go

@@ -3,6 +3,7 @@ package oscommands
 import (
 	"fmt"
 	"os"
+	"os/exec"
 	"path/filepath"
 )
 
@@ -22,3 +23,8 @@ func (c *OSCommand) UpdateWindowTitle() error {
 	argString := fmt.Sprint("title ", filepath.Base(path), " - Lazygit")
 	return c.Cmd.NewShell(argString, c.UserConfig().OS.ShellFunctionsFile).Run()
 }
+
+func TerminateProcessGracefully(cmd *exec.Cmd) error {
+	// Signals other than SIGKILL are not supported on Windows
+	return nil
+}

pkg/tasks/tasks.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"github.com/jesseduffield/gocui"
+	"github.com/jesseduffield/lazygit/pkg/commands/oscommands"
 	"github.com/jesseduffield/lazygit/pkg/utils"
 	"github.com/sasha-s/go-deadlock"
 	"github.com/sirupsen/logrus"
@@ -165,6 +166,17 @@ func (self *ViewBufferManager) NewCmdTask(start func() (*exec.Cmd, io.Reader), p
 				// and the user is flicking through a bunch of items.
 				self.throttle = time.Since(startTime) < THROTTLE_TIME && timeToStart > COMMAND_START_THRESHOLD
 
+				// Kill the still-running command. The only reason to do this is to save CPU usage
+				// when flicking through several very long diffs when diff.algorithm = histogram is
+				// being used, in which case multiple git processes continue to calculate expensive
+				// diffs in the background even though they have been stopped already.
+				//
+				// Unfortunately this will do nothing on Windows, so Windows users will have to live
+				// with the higher CPU usage.
+				if err := oscommands.TerminateProcessGracefully(cmd); err != nil {
+					self.Log.Errorf("error when trying to terminate cmd task: %v; Command: %v %v", err, cmd.Path, cmd.Args)
+				}
+
 				// close the task's stdout pipe (or the pty if we're using one) to make the command terminate
 				onDone()
 			}