magit-log-format-graph-function: has to be a function now

Previously the value could also be `nil',
now `identity' has to be used in place of that.
place refs before hash in verbose logs, don't abbreviate hash
refactor log washing

Washing verbose logs is very different from washing other logs; we
should not use the same function to do both.  Add the new function
`magit-log-wash-verbose' and take advantage of the its reduduced
complexity and improve `magit-log-wash-line' too.
consistently use "verbose" when referring to verbose log

Rename log type `long' to `verbose',
rename function `magit-insert-log-long' to `magit-insert-log-long',
rename variable `magit-log-long-re' to `magit-log-verbose-re'.
Merge branch 'n/wash-log' into next