fsck: support comments & empty lines in skipList

fsck: support comments & empty lines in skipList

It's annoying not to be able to put comments and empty lines in the
skipList, when e.g. keeping a big central list of commits to skip in
/etc/gitconfig, which was my motivation for 1362df0d41 ("fetch:
implement fetch.fsck.*", 2018-07-27).

Implement that, and document what version of Git this was changed in,
since this on-disk format can be expected to be used by multiple
versions of git.

There is no notable performance impact from this change, using the
test setup described a couple of commits back:

    Test                                             HEAD~             HEAD
    ----------------------------------------------------------------------------------------
    1450.3: fsck with 0 skipped bad commits          7.69(7.27+0.42)   7.86(7.48+0.37) +2.2%
    1450.5: fsck with 1 skipped bad commits          7.69(7.30+0.38)   7.83(7.47+0.36) +1.8%
    1450.7: fsck with 10 skipped bad commits         7.76(7.38+0.38)   7.79(7.38+0.41) +0.4%
    1450.9: fsck with 100 skipped bad commits        7.76(7.38+0.38)   7.74(7.36+0.38) -0.3%
    1450.11: fsck with 1000 skipped bad commits      7.71(7.30+0.41)   7.72(7.34+0.38) +0.1%
    1450.13: fsck with 10000 skipped bad commits     7.74(7.34+0.40)   7.72(7.34+0.38) -0.3%
    1450.15: fsck with 100000 skipped bad commits    7.75(7.40+0.35)   7.70(7.29+0.40) -0.6%
    1450.17: fsck with 1000000 skipped bad commits   7.12(6.86+0.26)   7.13(6.87+0.26) +0.1%

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>