Commit Inspect Shortcuts

Added gitlink-cli commit for read-only commit inspection.

Command	Endpoint
`commit +list`	`GET /v1/{owner}/{repo}/commits`
`commit +files`	`GET /v1/{owner}/{repo}/commits/{sha}/files`
`commit +diff`	`GET /v1/{owner}/{repo}/commits/{sha}/diff`
`commit +blame`	`GET /v1/{owner}/{repo}/blame`

Validation:

GOPROXY=https://goproxy.cn,direct go test ./shortcuts/commit ./shortcuts
go vet ./shortcuts/commit ./shortcuts
go run . commit --help