.gitlink for Summer of Code

[Eric Lesh <eclesh@ucla.edu>]

I would like to tackle .gitlink for Summer of Code.  The deadline is
about a day away, but this is a chance to make sure you still think
.gitlink is a good idea, and to see if there are any big problems with
the idea in general (before a more detailed proposal is actually submitted).

.gitlink is for a lightweight checkout of a branch into a separate
directory on the local filesystem.  A .gitlink'ed checkout has its own
index+HEAD, but otherwise refers back to the main repository for
objects, refs, etc.

Junio has said (http://www.spinics.net/lists/git/msg24964.html) he
works in a similar way with many work trees which are symlinked to his
main repo:
: gitster git.wk0; ls -l .git/
total 120
drwxrwsr-x  3 junio src  4096 Mar  5 16:22 ./
drwxrwsr-x 15 junio src 16384 Mar  5 16:23 ../
-rw-rw-r--  1 junio src    41 Mar  5 16:22 HEAD
lrwxrwxrwx  1 junio src    27 Mar  3 22:53 config -> /src/git/.git/config
lrwxrwxrwx  1 junio src    26 Mar  3 22:53 hooks -> /src/git/.git/hooks/
-rw-rw-r--  1 junio src 82455 Mar  5 16:22 index
lrwxrwxrwx  1 junio src    25 Mar  3 22:53 info -> /src/git/.git/info/
drwxrwsr-x  3 junio src  4096 Mar  3 22:59 logs/
lrwxrwxrwx  1 junio src    28 Mar  3 22:53 objects -> /src/git/.git/objects/
lrwxrwxrwx  1 junio src    32 Mar  3 22:53 packed-refs -> /src/git/.git/packed-refs
lrwxrwxrwx  1 junio src    25 Mar  3 22:53 refs -> /src/git/.git/refs/
lrwxrwxrwx  1 junio src    28 Mar  3 22:53 remotes -> /src/git/.git/remotes/
lrwxrwxrwx  1 junio src    29 Mar  3 22:53 rr-cache -> /src/git/.git/rr-cache/

A .gitlink could simplify this, ridding the directory of symlinks and
instead having a .gitlink file that specifies a $GIT_DIR to which
everything refers.

My implementation will be based on Josef Weidendorfer's "[RFC]
Lightweight checkouts via ".gitlink""
(http://thread.gmane.org/gmane.comp.version-control.git/33755), which
didn't receive much objection (or comment at all).  Hopefully this
doesn't mean you simply aren't interested.

Goals:
  o Lightweight checkouts are essentially branches that reside outside of the
    main repository on the local filesystem.
  o They act as normal git checkouts (i.e. git tools know about .gitlink)
  o They can be moved around and maintain their link to the base repo
  o They can exist within other checkouts (for eventual submodule support)
  o git-clone --light-weight option to set this up
  o porcelains (i.e. cogito) don't barf

Is this something that you would like to see?  Any other comments?

Thanks,

Eric