docs: don't talk about $GIT_DIR/refs/ everywhere

docs: don't talk about $GIT_DIR/refs/ everywhere

It is misleading to say that we pull refs from $GIT_DIR/refs/*, because we
may also consult the packed refs mechanism. These days we tend to treat
the "refs hierarchy" as more of an abstract namespace that happens to be
represented as $GIT_DIR/refs. At best, this is a minor inaccuracy, but at
worst it can confuse users who then look in $GIT_DIR/refs and find that it
is missing some of the refs they expected to see.

This patch drops most uses of "$GIT_DIR/refs/*", changing them into just
"refs/*", under the assumption that users can handle the concept of an
abstract refs namespace. There are a few things to note:

  - most cases just dropped the $GIT_DIR/ portion. But for cases where
    that left _just_ the word "refs", I changed it to "refs/" to help
    indicate that it was a hierarchy.  I didn't do the same for longer
    paths (e.g., "refs/heads" remained, instead of becoming
    "refs/heads/").

  - in some cases, no change was made, as the text was explicitly about
    unpacked refs (e.g., the discussion in git-pack-refs).

  - In some cases it made sense instead to note the existence of packed
    refs (e.g., in check-ref-format and rev-parse).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>