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/file/dir_with_untracked_file.go

@@ -8,7 +8,7 @@ import (
 var DirWithUntrackedFile = NewIntegrationTest(NewIntegrationTestArgs{
 	// notably, we currently _don't_ actually see the untracked file in the diff. Not sure how to get around that.
 	Description:  "When selecting a directory that contains an untracked file, we should not get an error",
-	ExtraCmdArgs: "",
+	ExtraCmdArgs: []string{},
 	Skip:         false,
 	SetupConfig:  func(config *config.AppConfig) {},
 	SetupRepo: func(shell *Shell) {

pkg/integration/tests/file/discard_changes.go

@@ -7,7 +7,7 @@ import (
 
 var DiscardChanges = NewIntegrationTest(NewIntegrationTestArgs{
 	Description:  "Discarding all possible permutations of changed files",
-	ExtraCmdArgs: "",
+	ExtraCmdArgs: []string{},
 	Skip:         true, // failing due to index.lock file being created
 	SetupConfig: func(config *config.AppConfig) {
 	},
@@ -51,7 +51,7 @@ var DiscardChanges = NewIntegrationTest(NewIntegrationTestArgs{
 		shell.RunShellCommand(`rm deleted-us.txt && git add deleted-us.txt`)
 		shell.RunShellCommand(`git commit -m "three"`)
 		shell.RunShellCommand(`git reset --hard conflict_second`)
-		shell.RunShellCommandExpectError(`git merge conflict`)
+		shell.RunCommandExpectError([]string{"git", "merge", "conflict"})
 
 		shell.RunShellCommand(`echo "new" > new.txt`)
 		shell.RunShellCommand(`echo "new staged" > new-staged.txt && git add new-staged.txt`)

pkg/integration/tests/file/discard_staged_changes.go

@@ -7,7 +7,7 @@ import (
 
 var DiscardStagedChanges = NewIntegrationTest(NewIntegrationTestArgs{
 	Description:  "Discarding staged changes",
-	ExtraCmdArgs: "",
+	ExtraCmdArgs: []string{},
 	Skip:         false,
 	SetupConfig: func(config *config.AppConfig) {
 	},

pkg/integration/tests/file/gitignore.go

@@ -7,7 +7,7 @@ import (
 
 var Gitignore = NewIntegrationTest(NewIntegrationTestArgs{
 	Description:  "Verify that we can't ignore the .gitignore file, then ignore/exclude other files",
-	ExtraCmdArgs: "",
+	ExtraCmdArgs: []string{},
 	Skip:         false,
 	SetupConfig: func(config *config.AppConfig) {
 	},

pkg/integration/tests/file/remember_commit_message_after_fail.go

@@ -14,13 +14,13 @@ fi
 
 var RememberCommitMessageAfterFail = NewIntegrationTest(NewIntegrationTestArgs{
 	Description:  "Verify that the commit message is remembered after a failed attempt at committing",
-	ExtraCmdArgs: "",
+	ExtraCmdArgs: []string{},
 	Skip:         false,
 	SetupConfig: func(config *config.AppConfig) {
 	},
 	SetupRepo: func(shell *Shell) {
 		shell.CreateFile(".git/hooks/pre-commit", preCommitHook)
-		shell.RunCommand("chmod +x .git/hooks/pre-commit")
+		shell.MakeExecutable(".git/hooks/pre-commit")
 
 		shell.CreateFileAndAdd("one", "one")