Given an object name, use open_istream() to get a git_istream handle
that you can read_istream() from as if you are using read(2) to read
the contents of the object, and close it with close_istream() when
you are done.

Currently, we do not do anything fancy--it just calls read_sha1_file()
and keeps the contents in memory as a whole, and carve it out as you
request with read_istream().

Signed-off-by: Junio C Hamano <gitster@pobox.com>