Have git SHA1 ID's changed in the last 5 years?

[Mark Polesky]

On Dec 17 2009, with commit #05eb689, I tracked down the
"birth commits" for the individual manuals in order to
establish the initial years for copyright purposes:

http://git.savannah.gnu.org/gitweb/?p=lilypond.git;a=commitdiff;h=05eb689

But now I see that none of the commit numbers I documented
point to any known commit objects:

`Music Glossary' was born 1999-10-04 with git commit 280a0bb
`Learning Manual' was born 1999-10-10 with git commit b9abaac
`Usage' was born 1999-10-10 with git commit c82c30c
`Notation Reference' was born 1999-10-11 with git commit 940dda0
`Internals Reference' was born 2000-10-21 with git commit 01e371f
`Essay' was born 2002-06-03 with git commit e38f5fc
`Extending' was born 2003-04-23 with git commit c08f6e8
`Contributor's Guide' was born 2007-09-15 with git commit 48f3356
`Web' was imported 2009-08-05 with git commit b938d71

I find it hard to believe that I not only got them all
wrong, but also that I listed 9 commits which don't even
refer to anything.  I'm happy to redo this old patch, but
can anyone explain what's going on here?

Thanks,
Mark