Add live tests for some methods in the git package

We relied too much on stubs for these methods. These new tests actually
invoke `git` commands in the context of a test repository.

git/fixtures/.gitignore

@@ -0,0 +1 @@
+*.git/COMMIT_EDITMSG

git/fixtures/simple.git/HEAD

@@ -0,0 +1 @@
+ref: refs/heads/main

git/fixtures/simple.git/config

@@ -0,0 +1,9 @@
+[core]
+	repositoryformatversion = 0
+	filemode = true
+	;bare = true
+	ignorecase = true
+	precomposeunicode = true
+[user]
+	name = Mona the Cat
+	email = monalisa@github.com

git/fixtures/simple.git/logs/HEAD

@@ -0,0 +1,2 @@
+0000000000000000000000000000000000000000 d1e0abfb7d158ed544a202a6958c62d4fc22e12f Mona the Cat <monalisa@github.com> 1614174263 +0100	commit (initial): Initial commit
+d1e0abfb7d158ed544a202a6958c62d4fc22e12f 6f1a2405cace1633d89a79c74c65f22fe78f9659 Mona the Cat <monalisa@github.com> 1614174275 +0100	commit: Second commit

git/fixtures/simple.git/logs/refs/heads/main

@@ -0,0 +1,2 @@
+0000000000000000000000000000000000000000 d1e0abfb7d158ed544a202a6958c62d4fc22e12f Mona the Cat <monalisa@github.com> 1614174263 +0100	commit (initial): Initial commit
+d1e0abfb7d158ed544a202a6958c62d4fc22e12f 6f1a2405cace1633d89a79c74c65f22fe78f9659 Mona the Cat <monalisa@github.com> 1614174275 +0100	commit: Second commit

git/fixtures/simple.git/objects/d1/e0abfb7d158ed544a202a6958c62d4fc22e12f

@@ -0,0 +1,3 @@
+x
²NK
+б0t²S╪╫ ILc
+"┌+Н╪@▓╬≤MМК╒╥7"^@≤мС▀╣bпZНxF°Н├h█▌аbХ╫╢;▓l╞²K
яТр©r╝3<ЙД│3бм3°Kc#-ЧЗ"нk8дZ.═╛2Йd╢=б^*)ESш&Цiq÷┬и▐П╜Б≥i│├о▀P┤
j┌╡A╒yА÷И
3*H/

git/fixtures/simple.git/refs/heads/main

@@ -0,0 +1 @@
+6f1a2405cace1633d89a79c74c65f22fe78f9659

git/git_test.go

@@ -1,12 +1,47 @@
 package git
 
 import (
+	"os"
 	"reflect"
 	"testing"
 
 	"github.com/cli/cli/internal/run"
 )
 
+func setGitDir(t *testing.T, dir string) {
+	// TODO: also set XDG_CONFIG_HOME, GIT_CONFIG_NOSYSTEM
+	old_GIT_DIR := os.Getenv("GIT_DIR")
+	os.Setenv("GIT_DIR", dir)
+	t.Cleanup(func() {
+		os.Setenv("GIT_DIR", old_GIT_DIR)
+	})
+}
+
+func TestLastCommit(t *testing.T) {
+	setGitDir(t, "./fixtures/simple.git")
+	c, err := LastCommit()
+	if err != nil {
+		t.Fatalf("LastCommit error: %v", err)
+	}
+	if c.Sha != "6f1a2405cace1633d89a79c74c65f22fe78f9659" {
+		t.Errorf("expected sha %q, got %q", "6f1a2405cace1633d89a79c74c65f22fe78f9659", c.Sha)
+	}
+	if c.Title != "Second commit" {
+		t.Errorf("expected title %q, got %q", "Second commit", c.Title)
+	}
+}
+
+func TestCommitBody(t *testing.T) {
+	setGitDir(t, "./fixtures/simple.git")
+	body, err := CommitBody("6f1a2405cace1633d89a79c74c65f22fe78f9659")
+	if err != nil {
+		t.Fatalf("CommitBody error: %v", err)
+	}
+	if body != "I'm starting to get the hang of things\n" {
+		t.Errorf("expected %q, got %q", "I'm starting to get the hang of things\n", body)
+	}
+}
+
 func Test_UncommittedChangeCount(t *testing.T) {
 	type c struct {
 		Label    string