refs: add GIT_TRACE_REFS debugging mechanism

refs: add GIT_TRACE_REFS debugging mechanism

When set in the environment, GIT_TRACE_REFS makes git print operations and
results as they flow through the ref storage backend. This helps debug
discrepancies between different ref backends.

Example:

    $ GIT_TRACE_REFS="1" ./git branch
    15:42:09.769631 refs/debug.c:26         ref_store for .git
    15:42:09.769681 refs/debug.c:249        read_raw_ref: HEAD: 0000000000000000000000000000000000000000 (=> refs/heads/ref-debug) type 1: 0
    15:42:09.769695 refs/debug.c:249        read_raw_ref: refs/heads/ref-debug: 3a238e539bcdfe3f9eb5010fd218640c1b499f7a (=> refs/heads/ref-debug) type 0: 0
    15:42:09.770282 refs/debug.c:233        ref_iterator_begin: refs/heads/ (0x1)
    15:42:09.770290 refs/debug.c:189        iterator_advance: refs/heads/b4 (0)
    15:42:09.770295 refs/debug.c:189        iterator_advance: refs/heads/branch3 (0)

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>