[PATCHv8 0/4] Support ref namespaces

This series adds support for dividing the refs of a single repository into multiple namespaces, each of which can have its own branches, tags, and HEAD. Git can expose each namespace as an independent repository to pull from and push to, while sharing the object store, and exposing all the refs to operations such as git-gc.

Storing multiple repositories as namespaces of a single repository avoids storing duplicate copies of the same objects, such as when storing multiple branches of the same source. The alternates mechanism provides similar support for avoiding duplicates, but alternates do not prevent duplication between new objects added to the repositories without ongoing maintenance, while namespaces do.

The first patch improves the prefix handling in the ref iteration functions, making it possible for us to implement for_each_namespaced_ref later. The next two patches implement and then use infrastructure for tracking the current namespace and iterating over the refs in that namespace. The last patch adds general documentation for namespaces, and specific references from the documentation on receive-pack, upload-pack, http-backend, and git.

v8: Fix error message in parsing of the --namespace option; noticed by Bert Wesarg.

v7 (unintentionally sent without a version number): Back out the change to actually use "refs/" as a filter, which caused t5501 to fail. Patch 1/4 now preserves the existing behavior, to avoid breaking assumptions like this; that seems preferable to tracking down all the places that would break due to this new ref filtering. Adding that filtering and fixing all the resulting breakage seems like an entirely separate change.

Josh Triplett and Jamey Sharp (4):
  Fix prefix handling in ref iteration functions
  Add infrastructure for ref namespaces
  Support ref namespaces for remote repositories via upload-pack and
    receive-pack
  Add documentation for ref namespaces

 Documentation/Makefile                 |    2 +-
 Documentation/git-http-backend.txt     |    8 +++
 Documentation/git-receive-pack.txt     |    2 +-
 Documentation/git-upload-pack.txt      |    4 ++
 Documentation/git.txt                  |   13 +++++-
 Documentation/gitnamespaces.txt        |   75 ++++++++++++++++++++++++++++++++
 builtin/receive-pack.c                 |   34 ++++++++++++---
 cache.h                                |    3 +
 contrib/completion/git-completion.bash |    3 +-
 environment.c                          |   41 +++++++++++++++++
 git.c                                  |   18 +++++++-
 refs.c                                 |   33 ++++++++++++--
 refs.h                                 |    3 +
 upload-pack.c                          |   15 +++---
 14 files changed, 230 insertions(+), 24 deletions(-)
 create mode 100644 Documentation/gitnamespaces.txt