Extending "extended SHA1" syntax to traverse through gitlinks?

[Josh Triplett <josh@joshtriplett.org>]

Currently, if you have a branch "somebranch" that contains a gitlink
"somecommit", you can write "somebranch:somecommit" to refer to the
commit, just like a tree or blob.  ("man git-rev-parse" defines this
syntax in the "SPECIFYING REVISIONS" section.)  You can use this
anywhere you can use a committish, including "git show
somebranch:somecommit", "git log somebranch:somecommit..anotherbranch",
or even "git format-patch -1 somebranch:somecommit".

However, you cannot traverse *through* the gitlink to look at files
inside its own tree, or to look at other commits relative to that
commit.  For instance, "somebranch:somecommit:somefile" and
"somebranch:somecommit~3" do not work.

I'd love to have a syntax that allows traversing through the gitlink to
other files or commits.  Ideally, I'd suggest the syntax above, as a
natural extension of the existing extended syntax.

(That syntax would potentially introduce ambiguity if you had a file
named "somecommit:somefile" or "somecommit~3".  That doesn't seem like a
problem, though; the existing syntax already doesn't support accessing a
file named "x..y" or "x...y", so scripts already can't expect to access
arbitrary filenames with that syntax without some kind of quoting, wich
we also don't have.)

Does this seem reasonable?  Would a patch introducing such syntax
(including documentation and tests) be acceptable?

- Josh Triplett