Introduce mgit save-hashes command

[pjasiun]

mgit update set versions in packages folder to the versions defined in mgit.json file. But the opposite command would be also needed, to set versions in mgit.json to hashes in the packages.

I propose the mgit save-hashes command which goes into each repository check the current hash and add #hash to each entry in mgit.json. It's very useful if the project uses fixed versions of dependencies and update them manually.

[pomek]

It can be also helpful for the tool for saving revisions in CKE5.

At this moment, we have to do it manually - https://github.com/ckeditor/ckeditor5-dev/blob/t/ckeditor5-revisions/1/packages/ckeditor5-dev-env/bin/save-revision.js.

[pomek]

Looks like work as I expected: ckeditor/ckeditor5@b55508d

[pomek]

I would call this command dump because this reminds me with the database dump.

[Reinmar]

Turns out I wanted to name it exactly like PJ proposed: #38 (comment) ;) So better to stick to that.

[pomek]

Ok…