The check-doc linter needs to be run when some kind of files are
modified (*.h or *.cpp), but only needs to run once if one of these
files is modified because it parses the whole tree at each call.
This diffs adds a new .runonce.arclint configuration file which
contains linters to be run only once per arc lint call. The
check-doc linter is moved from the .arclint file to this new
This speeds up arc lint on large diffs.
arc lint --everything now takes 0'34" on my machine (was 2'22").
Depends on D3299