Add block-sha1 in favour of the mozilla routines

Add block-sha1 in favour of the mozilla routines

Since block-sha1 from git.git has such excellent performance, we
can also get rid of the openssl dependency. It's rather simple
to add it back later as an optional extra, but we really needn't
bother to pull in the entire ssl library and have to deal with
linking issues now that we have the portable and, performance-wise,
truly excellent block-sha1 code to fall back on.

Since this requires a slight revamp of the build rules anyway, we
take the opportunity to fix including EXTRA_OBJS in the final build
as well.

The block-sha1 code was originally implemented for git.git by
Linus Torvalds <torvalds@linux-foundation.org> and was later
polished by Nicolas Pitre <nico@cam.org>.

Signed-off-by: Andreas Ericsson <ae@op5.se>