Add a command to clone Git repositories

[akirak]

I plan on adding a command for cloning Git repositories to the local machine.

The name will be git-identity-clone. There is magit-clone, but with git-identity-clone, you will be able to clone remote repositories to designated directories by default.

To implement this feature, the format of git-identity-list will be changed. Each item in the list will get an optional :targets field, which consists of a URL spec, a local path, and options. This field lets you define mappings between remote URLs and local directories.

A few notes on additional features:

• It would be better if a corresponding identity is automatically set when a repository is cloned by the command. The current implementation of magit-clone does not seem to allow a custom clone hook, so I have to implement a git-clone wrapper on my own, or maybe contribute to magit to allow this feature.
• By adding support for forges such GitHub and GitLab via APIs, it would be possible to list remote repositories as well as create a remote repository. This feature is of less importance, though.

This is currently work in progress at feature/clone branch.

[akirak]

forge has added support for a similar feature, but I'm willing to continue with this.

[akirak]

I am not sure if I should support this feature in this package.

[akirak]

I've found another package that should support the feature of cloning git repositories. I will close this PR.