refs: don't normalize log messages with `REF_SKIP_CREATE_REFLOG`

refs: don't normalize log messages with `REF_SKIP_CREATE_REFLOG`

When the `REF_SKIP_CREATE_REFLOG` flag is set we skip the creation of
the reflog entry, but we still normalize the reflog message when we
queue the update. This is a waste of resources as the normalized message
will never get used in the first place.

Fix this issue by skipping the normalization in case the flag is set.
This leads to a surprisingly large speedup when migrating from the
"files" to the "reftable" backend:

    Benchmark 1: migrate files:reftable (refcount = 1000000, revision = HEAD~)
      Time (mean ± σ):     878.5 ms ±  14.9 ms    [User: 726.5 ms, System: 139.2 ms]
      Range (min … max):   858.4 ms … 941.3 ms    50 runs

    Benchmark 2: migrate files:reftable (refcount = 1000000, revision = HEAD)
      Time (mean ± σ):     831.1 ms ±  10.5 ms    [User: 694.1 ms, System: 126.3 ms]
      Range (min … max):   812.4 ms … 851.4 ms    50 runs

    Summary
      migrate files:reftable (refcount = 1000000, revision = HEAD) ran
        1.06 ± 0.02 times faster than migrate files:reftable (refcount = 1000000, revision = HEAD~)

And an ever larger speedup when migrating the other way round:

    Benchmark 1: migrate reftable:files (refcount = 1000000, revision = HEAD~)
      Time (mean ± σ):     923.6 ms ±  11.6 ms    [User: 705.5 ms, System: 208.1 ms]
      Range (min … max):   905.3 ms … 946.5 ms    50 runs

    Benchmark 2: migrate reftable:files (refcount = 1000000, revision = HEAD)
      Time (mean ± σ):     818.5 ms ±   9.0 ms    [User: 627.6 ms, System: 180.6 ms]
      Range (min … max):   802.2 ms … 842.9 ms    50 runs

    Summary
      migrate reftable:files (refcount = 1000000, revision = HEAD) ran
        1.13 ± 0.02 times faster than migrate reftable:files (refcount = 1000000, revision = HEAD~)

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>