proof of concent for push to git bundles with MANIFEST

This is a shell script, so not final code, and it does not use git-annex
at all, but it shows how to push to git bundles, listed in a MANIFEST,
the same as the git-remote-annex program will eventually do.

While developing this, I realized that the design needed to be changed
slightly regarding where refs are stored. Since a push can delete a ref
from a remote, storing each newly pushed ref in a bundle won't work,
because deleting a ref would then entail deleting all old bundles and
re-uploading from scratch. So instead, only the refs in the last bundle
listed in the MANIFEST are the active refs. Any refs in prior bundles
are just old refs that were stored previously (a reflog as it were).

That means that, in a situation where two different people are pushing
to the same special remote from different repos, whoever pushes last
wins. Any refs pushed by the other person earlier will be ignored. This
may not be desirable, and git-annex might be able use the git-annex
branch to detect such situations and rescue the refs that got lost. Even
without such a recovery process though, the refs that the other person
thought they pushed will be preserved in their refs/namespaces/mine, so
a pull followed by a push will generally resolve the situation.

Note that the use of refs/namespaces/mine in the bundle is not really
desirable, and it might be worth making a local clone of the repo in
order to set up the refs that will be put in the bundle. Which seems to
be the only way to avoid needing that. But it does need to maintain
the refs/namespaces/mine/ in the git repo in order to remember what refs
have been pushed to the remote before, in order to include them in the
next bundle pushed. A name that includes the remote uuid will be needed
in the final implementation.

Anyway, this shell script seems to fully work, including incremental
pushing, force pushing, and pushes that delete refs.

Sponsored-by: Brett Eisenberg on Patreon

doc/internals/git-remote-annex.mdwn

@@ -1,6 +1,6 @@
 This adds two new object types to git-annex, GITMANIFEST and a GITBUNDLE.
 
-GITMANIFEST-$UUID is the manifest for a git repository stored in the
+GITMANIFEST--$UUID is the manifest for a git repository stored in the
 git-annex repository with that UUID.
 
 GITBUNDLE--sha256 is a git bundle.
@@ -9,16 +9,21 @@ GITBUNDLE--sha256 is a git bundle.
 
 An ordered list of bundle keys, one per line.
 
+The last bundle in the list provides all refs that are currently stored in
+the repository. The bundles before it in the list can incrementally provide
+objects, but not refs.
+
 # fetching
 
 1. download GITMANIFEST for the uuid of the special remote
 2. download each listed GITBUNDLE object that we don't have
-3. git fetch from bundles in timestamp order
+3. `git bundle unpack` each bundle in order
+4. `git fetch` from the last bundle listed in the manifest
 
 # pushing (incrementally)
 
-1. create git bundle containing refs to push, and objects since
-   the previously pushed refs
+1. create git bundle all refs that will be stored in the repository, 
+   and objects since the previously pushed refs
 2. hash to calculate GITBUNDLE key
 3. upload GITBUNDLE object
 4. download current manifest
@@ -26,7 +31,8 @@ An ordered list of bundle keys, one per line.
 
 # pushing (replacing incrementals with single bundle)
 
-1. create git bundle containing refs to push and all objects
+1. create git bundle containing all refs stored in the repository, and all
+   objects
 2. hash to calculate GITBUNDLE object name
 3. upload GITBUNDLE object
 4. download current manifest