Enable updates on GitHub-based dependencies

[digorithm]

Problem

Forc now allows users to specify remote dependencies hosted on GitHub (#101), however, once a dependency at a given reference (either default, branch, or tag) has been downloaded, there's no way to check if there are newer updates to it (i.e fresher commits on top of a reference) and actually perform the updates.

Proposal

Add another command to Forc's CLI, update, that will check for updates on the dependencies in Forc.toml that have git set and perform these updates; downloading the new one and replacing the old one.

forc update should:

• If the dependency doesn't have a branch/tag specified, check if there are newer commits on the default branch.
• If it has a branch specified, check if there are newer commits on the branch specified.
• if it has a tag specified, check if there is a newer tag based on semver.
• In case of a hash diff [1], download a new tarball of the updated dependency and replace the previous one with it.

Non-functional requirements needed:

• Dependency-related code starting to duplicate across forc_build and (the new) forc_update. This needs to be refactored.

[1]: We store the hash in the name of the dependency's locally installed directory

[adlerjohn]

I would say for the second one, instead of

if it has a branch/tag specified, check if there are newer commits on the branch/tag specified

It should be

if it has a branch specified, check if there are newer commits on the branch specified
if it has a tag specified, check if there is a newer tag based on semver

[digorithm]

I would say for the second one, instead of

if it has a branch/tag specified, check if there are newer commits on the branch/tag specified

It should be

if it has a branch specified, check if there are newer commits on the branch specified
if it has a tag specified, check if there is a newer tag based on semver

Perfect! Updated the description.