Fix DAGMan problems on upgrade from 7.8 #3707

Revise note about problems with DAGMan on upgrade. Add version history
Make changes from Kent's review #3707

Make the following changes:

	Nathan, can you remove the update notes commit?  That has to go
	into v8.0.0 so it can get immediately pushed out to the web.  I send a
	modified version of your text to Karen, and she's going to take care of
	that end of things.  But I guess if the change is still on the
	V8_0-gittrac_3707-branch I think it will cause a conflict at some
	point...

	I'd prefer the structure to be like this:

	  if (!submitFileVersion.built_since_version(7,9,1)) {
		...
	  } else {
		// Check for existence of default log
	  }

	because I think checking the submit file version is a little
	cleaner; and we can have the else because if the submit file version
	check "fails" we don't need to check for the existence of the default
	log file.
Gittrac #3707:  Just added a comment to clarify one aspect of the new
code.
Merge remote-tracking branch 'origin/V8_0-gittrac_3707-branch' into V8_0-branch

Conflicts:
	doc/version-history/8-0.history.tex
Merge branch 'V8_0-branch'