RFC: git cat-file --follow-symlinks?

[David Turner <dturner@twopensource.com>]

I recently had a situation where I was using git cat-file (--batch) to
read files and directories out of the repository -- basically, todo the
equivalent of open, opendir, etc, on an arbitrary revision.
Unfortunately, I had to do a lot of gymnastics to handle symlinks in the
repository.  Instead of just doing echo $SHA:foo/bar/baz | git cat-file
--batch, I would have to first check if foo was a symlink, and if so,
follow it, and then check bar, and so on.

Instead, it would be cool if cat-file had a mode in which it would
follow symlinks.

The major wrinkle is that symlinks can point outside the repository --
either because they are absolute paths, or because they are relative
paths with enough ../ in them.  For this case, I propose that
--follow-symlinks should output [sha] "symlink" [target] instead of the
usual [sha] "blob" [bytes].  Since --follow-symlinks is new, this format
change will not break any existing code.

(I also propose that we use Linux's limit of 40 symlinks by default, but
--follow-symlinks could also have =max_links_to_follow to adjust this if
anyone cares)

Do people think this is reasonable?  If so, I'll see if I can get some
time to work on it this month.