diff-merges: avoid history simplifications when diffing merges

Doing diffs for merges are special; they should typically avoid history
simplification.  For example, with

    git log --diff-merges=first-parent -- path

the default history simplification would remove merge commits from
consideration if the file "path" matched the second parent.  That is
counter to what the user wants when looking for first-parent diffs.
Similar comments can be made for --diff-merges=separate (which diffs
against both parents) and --diff-merges=remerge (which diffs against a
remerge of the merge commit).

However, history simplification still makes sense if not doing diffing
merges, and it also makes sense for the combined and dense-combined
forms of diffing merges (because both of those are defined to only show
a diff when the merge result at the relevant paths differs from *both*
parents).

So, for separate, first-parent, and remerge styles of diff-merges, turn
off history simplification.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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