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_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
+}