[RFC] Importing from a patch-oriented SCM

I am drafting an import script to turn a GNU Arch into a GIT archive. Importing the branches and commits increamentally is reasonably straightforward -- or so it seems so far. Note: the repository manipulation is based on cvsimport -- so my knowledge of the git repo internals is still pertty close to zero.

Each patchset has a unique identifier, and can carry metadata with the identifiers of the patches it "includes". If you are using gnu arch, when you merge across branches, it'll know to skip a particular patchset if it has been applied already. AFAICT there is no such concept in GIT, and I wonder what to do with all this metadata about merges.

My proto-plan is to keep track of merged stuff (in a cache file somewhere), and if a particular merge means that the branches are fully merged up to the last patch of the series (if no commits from the source branch have been skipped) mark it as a merge in GIT.

If the merges have been done out-of-order, that may show up in the latest merge. For example, branch A and B of the same project each have 10 commits from the branching point. If a merge A -> B does commits 1,2,3,7,8 it gets imported to git as a merge up to commit "3", although there is more there. The next merge, which does 4,5,6,10 will show up as a merge of commit 8.

Yuk. 

If I remember correctly, Junio added some stuff in the merge & rebase code that will identify if a particular patch has been seen and applied, and skip it even if it's a bit out of order. But I don't know what that is based on, and whether I can somehow maximize the chances of the patch being identified as already merged across branches. If it's based on the initial commit identifier being carried through (does that travel with commits when you merge?) I stand a small chance. Otherwise, I'm lost.

Suggestions? 

cheers,

martin