git log - merges since whenever

gistfile1.txt

git log --merges --first-parent master \
        --pretty=format:"%h %<(10,trunc)%aN %C(white)%<(15)%ar%Creset %C(red bold)%<(15)%D%Creset %s"
Explaining each argument:

--merges: only "merge" commits (more than 1 parent);
--first-parent master: only merges applied to master. This removes the entries where someone merged master into their branches;
--pretty-format: applies the following formatting:
%h: the commit short hash;
%<(10,trunc)%aN: author name, truncated at 10 chars;
%<(15)%ar: the relative commit time, padded to 15 chars;
%<(15)%D: the tag names, also padded to 15 chars;
%s: first line of the commit message.