Improve ABC repository management in Makefile

Improve ABC repository management in Makefile

`rev-parse --short` output may have a different abbreviated hash length than
ABCREV, so a simple string comparison always fails, even if the correct
commit is checked out. Pass both commits through rev-parse and then
compare the full hashes instead.

Add an `echo-abc-rev` target so that packaging scripts can set ABCPULL=0 and
handle all the git nastiness themselves.