"segmentation violation" issue on executing generate command

Hey ! I got an issue by running `commitlog generate` after a fresh install

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x102fecdbc]

goroutine 1 [running]:
github.com/barelyhuman/commitlog/pkg.(*Generator).readCommitsInTags(0x140001b2140)
/root/go/pkg/mod/github.com/barelyhuman/commitlog@v0.0.11-0.20220919094556-faf22fe8c91a/pkg/generator.go:91 +0x1bc
github.com/barelyhuman/commitlog/pkg.(*Generator).ReadCommmits(0x140001b2140)
/root/go/pkg/mod/github.com/barelyhuman/commitlog@v0.0.11-0.20220919094556-faf22fe8c91a/pkg/generator.go:175 +0x4c
github.com/barelyhuman/commitlog/commands.Commitlog(0x14000136230?)
/root/go/pkg/mod/github.com/barelyhuman/commitlog@v0.0.11-0.20220919094556-faf22fe8c91a/commands/commitlog.go:57 +0x538
main.main.func2(0x14000000a20?)
/root/go/pkg/mod/github.com/barelyhuman/commitlog@v0.0.11-0.20220919094556-faf22fe8c91a/main.go:32 +0x1c
github.com/urfave/cli/v2.(*Command).Run(0x14000000a20, 0x1400010d840)
/root/go/pkg/mod/github.com/urfave/cli/v2@v2.8.1/command.go:169 +0x4f8
github.com/urfave/cli/v2.(*App).RunContext(0x14000140680, {0x1031ffb90?, 0x103551e20}, {0x1400001e040, 0x2, 0x2})
/root/go/pkg/mod/github.com/urfave/cli/v2@v2.8.1/app.go:341 +0x85c
github.com/urfave/cli/v2.(*App).Run(...)
/root/go/pkg/mod/github.com/urfave/cli/v2@v2.8.1/app.go:247
main.main()
/root/go/pkg/mod/github.com/barelyhuman/commitlog@v0.0.11-0.20220919094556-faf22fe8c91a/main.go:143 +0xc60
```

在全新安装后运行 `commitlog generate` 以重现崩溃，然后检查 `pkg/generator.go` 第 91 行的 `Generator.readCommitsInTags` 及其来自 `ReadCommmits` 的调用。跟踪来自 `commands/commitlog.go` 的输入，并确认在所报告的设置下该命令完成时不会发生 nil-pointer panic。

由索引模型根据 Issue 内容生成。