version control - What are .git/info/grafts for?

Question

I am trying to figure out what is the 'grafts' in the Git.

For example, in one of the latest comments here, Tobu suppose to use git-filter-branch and .git/info/grafts to join two repositories.

But I don't understand why I need these grafts? It seems, that all work without last two commands.

Answer 1

From Git Wiki:

Graft points or grafts enable two otherwise different lines of development to be joined together. It works by letting users record fake ancestry information for commits. This way you can make git pretend the set of parents a commit has is different from what was recorded when the commit was created.

Reasons for Using Grafts

Grafts can be useful when moving development to git, since it allows you to make cloning of the old history imported from another SCM optional. This keeps the initial clone for users who just wants to follow the latest version down while developers can have the full development history available.

When Linus started using git for maintaining his kernel tree there didn't exist any tools to convert the old kernel history. Later, when the old kernel history was imported into git from the bkcvs gateway, grafts was created as a method for making it possible to tie the two different repositories together.