[RFD] not grep but not log -S

[Junio C Hamano <gitster@pobox.com>]

If you have a series of commits that record the contents of "what's
cooking" reports (i.e., "git log origin/todo -- whats-cooking.txt" in
your clone of this project), and have a specific topic name you are
interested in (e.g., "jc/test-set-e-clean"), what's the easiest way
to find out in which issues of the "what's cooking" report the topic
appeared?

   $ git log -Sjc/test-set-e-clean -p whats-cooking.txt

would tell you when it appeared and when it went away, but does not
mention anything about the (potentially) multiple issues the topic
stayed in the report in between.  Git being a tool to track contents
but is primarily used to track _changes_ in contents, this may be
unavoidable, but it is not very satisfying.

Looking for the string in each and every version, e.g.,

    $ git grep jc/test-set-e-clean $(printf "HEAD~%d " $(seq 0 20)) \
			whats-cooking.txt

may give something close to what I want, but it makes me feel dirty
that I have to _guess_ how many is sufficient.