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/integration/tests/commit/commit_wip_with_prefix.go

@@ -7,7 +7,7 @@ import (
 
 var CommitWipWithPrefix = NewIntegrationTest(NewIntegrationTestArgs{
 	Description:  "Commit with skip hook and config commitPrefix is defined. Prefix is ignored when creating WIP commits.",
-	ExtraCmdArgs: "",
+	ExtraCmdArgs: []string{},
 	Skip:         false,
 	SetupConfig: func(testConfig *config.AppConfig) {
 		testConfig.UserConfig.Git.CommitPrefixes = map[string]config.CommitPrefixConfig{"repo": {Pattern: "^\\w+\\/(\\w+-\\w+).*", Replace: "[$1]: "}}