magit-{refresh-log-buffer,insert-bisect-rest}: drop --abbrev-commit

It is not required because we explicitly use "%h" for oneline
logs and for verbose logs this argument it is ignored anyway.
log: respect core.abbrev

Remove the variable `magit-sha1-abbrev-length' and instead just respect
the git variable `core.abbrev'.  This allows users to set this per
repository.

Also remove the function `magit-diff-abbrev-arg' which used the value
of the option and returned "--abbrev=<N>".  Beside that, the only place
where the elisp variable was used was when aligning lines in logs that
contained only part of the graph.  Because we move the hash to the
beginning of the line we have to align based on the default length of
the hash.  We can get that by using `git config core.abbrev' once per
log, so it's cheap.
magit-log-count: limit scope

We only cut of logs after a certain number of commits when they are
shown in a log buffer, other logs usually are not long enough for that
to be necessary anyway.  Nevertheless, we now establish the dynamically
bound counter in `magit-wash-log', which is used for all logs.  Doing
so is cheap because it only amounts to calling `cl-incf' N times, where
N is usually fairly low.