Merge branch 'so/log-diff-merges-opt'

Earlier, to countermand the implicit "-m" option when the "--first-parent" option is used with "git log", we added the "--[no-]diff-merges" option in the jk/log-fp-implies-m topic. To leave the door open to allow the "--diff-merges" option to take values that instructs how patches for merge commits should be computed (e.g. "cc"? "-p against first parent?"), redefine "--diff-merges" to take non-optional value, and implement "off" that means the same thing as "--no-diff-merges". * so/log-diff-merges-opt: t/t4013: add test for --diff-merges=off doc/git-log: describe --diff-merges=off revision: change "--diff-merges" option to require parameter