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/custom.go

@@ -1,5 +1,7 @@
 package git_commands
 
+import "github.com/mgutz/str"
+
 type CustomCommands struct {
 	*GitCommon
 }
@@ -14,5 +16,5 @@ func NewCustomCommands(gitCommon *GitCommon) *CustomCommands {
 // If you want to run a new command, try finding a place for it in one of the neighbouring
 // files, or creating a new BlahCommands struct to hold it.
 func (self *CustomCommands) RunWithOutput(cmdStr string) (string, error) {
-	return self.cmd.New(cmdStr).RunWithOutput()
+	return self.cmd.New(str.ToArgv(cmdStr)).RunWithOutput()
 }