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/git_command_builder.go

@@ -49,6 +49,10 @@ func (self *GitCommandBuilder) RepoPath(value string) *GitCommandBuilder {
 	return self
 }
 
-func (self *GitCommandBuilder) ToString() string {
-	return "git " + strings.Join(self.args, " ")
+func (self *GitCommandBuilder) ToArgv() []string {
+	return append([]string{"git"}, self.args...)
+}
+
+func (self *GitCommandBuilder) ToString() string {
+	return strings.Join(self.ToArgv(), " ")
 }