This avoids hardcoding the sha size, so when git uses sha256, it will
output the full sha256 and not a truncation to 40 characters.
I reviewed git's history, and while there have been some
bugs with commands not supporting --no-abbrev (eg git diff --no-index
--no-abbrev was broken in git 2.1), none of the commands git-annex
uses will be impacted by those old bugs.