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/file_loader_test.go

@@ -20,14 +20,13 @@ func TestFileGetStatusFiles(t *testing.T) {
 		{
 			"No files found",
 			oscommands.NewFakeRunner(t).
-				Expect(`git status --untracked-files=yes --porcelain -z`, "", nil),
+				ExpectGitArgs([]string{"status", "--untracked-files=yes", "--porcelain", "-z"}, "", nil),
 			[]*models.File{},
 		},
 		{
 			"Several files found",
 			oscommands.NewFakeRunner(t).
-				Expect(
-					`git status --untracked-files=yes --porcelain -z`,
+				ExpectGitArgs([]string{"status", "--untracked-files=yes", "--porcelain", "-z"},
 					"MM file1.txt\x00A  file3.txt\x00AM file2.txt\x00?? file4.txt\x00UU file5.txt",
 					nil,
 				),
@@ -102,7 +101,7 @@ func TestFileGetStatusFiles(t *testing.T) {
 		{
 			"File with new line char",
 			oscommands.NewFakeRunner(t).
-				Expect(`git status --untracked-files=yes --porcelain -z`, "MM a\nb.txt", nil),
+				ExpectGitArgs([]string{"status", "--untracked-files=yes", "--porcelain", "-z"}, "MM a\nb.txt", nil),
 			[]*models.File{
 				{
 					Name:                    "a\nb.txt",
@@ -122,8 +121,7 @@ func TestFileGetStatusFiles(t *testing.T) {
 		{
 			"Renamed files",
 			oscommands.NewFakeRunner(t).
-				Expect(
-					`git status --untracked-files=yes --porcelain -z`,
+				ExpectGitArgs([]string{"status", "--untracked-files=yes", "--porcelain", "-z"},
 					"R  after1.txt\x00before1.txt\x00RM after2.txt\x00before2.txt",
 					nil,
 				),
@@ -161,8 +159,7 @@ func TestFileGetStatusFiles(t *testing.T) {
 		{
 			"File with arrow in name",
 			oscommands.NewFakeRunner(t).
-				Expect(
-					`git status --untracked-files=yes --porcelain -z`,
+				ExpectGitArgs([]string{"status", "--untracked-files=yes", "--porcelain", "-z"},
 					`?? a -> b.txt`,
 					nil,
 				),