Construct arg vector manually rather than parse string

By constructing an arg vector manually, we no longer need to quote arguments

Mandate that args must be passed when building a command

Now you need to provide an args array when building a command.
There are a handful of places where we need to deal with a string,
such as with user-defined custom commands, and for those we now require
that at the callsite they use str.ToArgv to do that. I don't want
to provide a method out of the box for it because I want to discourage its
use.

For some reason we were invoking a command through a shell when amending a
commit, and I don't believe we needed to do that as there was nothing user-
supplied about the command. So I've switched to using a regular command out-
side the shell there

pkg/commands/git_commands/diff.go

@@ -0,0 +1,19 @@
+package git_commands
+
+import "github.com/jesseduffield/lazygit/pkg/commands/oscommands"
+
+type DiffCommands struct {
+	*GitCommon
+}
+
+func NewDiffCommands(gitCommon *GitCommon) *DiffCommands {
+	return &DiffCommands{
+		GitCommon: gitCommon,
+	}
+}
+
+func (self *DiffCommands) DiffCmdObj(diffArgs []string) oscommands.ICmdObj {
+	return self.cmd.New(
+		NewGitCmd("diff").Arg("--submodule", "--no-ext-diff", "--color").Arg(diffArgs...).ToArgv(),
+	)
+}