A case where diff.colorMoved=plain is more sensible than diff.colorMoved=zebra & others

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Let's ignore how bad this patch is for git.git, and just focus on how
diff.colorMoved treats it:

    diff --git a/builtin/add.c b/builtin/add.c
    index f65c172299..d1155322ef 100644
    --- a/builtin/add.c
    +++ b/builtin/add.c
    @@ -6,5 +6,3 @@
     #include "cache.h"
    -#include "config.h"
     #include "builtin.h"
    -#include "lockfile.h"
     #include "dir.h"
    diff --git a/builtin/am.c b/builtin/am.c
    index 8f27f3375b..eded15aa8a 100644
    --- a/builtin/am.c
    +++ b/builtin/am.c
    @@ -6,3 +6,2 @@
     #include "cache.h"
    -#include "config.h"
     #include "builtin.h"
    diff --git a/builtin/blame.c b/builtin/blame.c
    index 06a7163ffe..44a754f190 100644
    --- a/builtin/blame.c
    +++ b/builtin/blame.c
    @@ -8,3 +8,2 @@
     #include "cache.h"
    -#include "config.h"
     #include "color.h"
    diff --git a/cache.h b/cache.h
    index ca36b44ee0..ea8d60b94a 100644
    --- a/cache.h
    +++ b/cache.h
    @@ -4,2 +4,4 @@
     #include "git-compat-util.h"
    +#include "config.h"
    +#include "new.h"
     #include "strbuf.h"

This is a common thing that's useful to have highlighted, e.g. we move
includes of config.h to some common file, so I want to se all the
deleted config.h lines as moved into the cache.h line, and then the
"lockfile.h" I removed while I was at it plain remove, and the new
"new.h" plain added.

Exactly that is what you get with diff.colorMoved=plain, but the default
of diff.colorMoved=zebra gets confused by this and highlights no moves
at all, same or "blocks" and "dimmed-zebra".

So at first I thought this had something to do with the many->one
detection, but it seems to be simpler, we just don't detect a move of
1-line with anything but plain, e.g. this works as expected in all modes
and detects the many->one:

    diff --git a/builtin/add.c b/builtin/add.c
    index f65c172299..f4fda75890 100644
    --- a/builtin/add.c
    +++ b/builtin/add.c
    @@ -5,4 +5,2 @@
      */
    -#include "cache.h"
    -#include "config.h"
     #include "builtin.h"
    diff --git a/builtin/branch.c b/builtin/branch.c
    index 0c55f7f065..52e39924d3 100644
    --- a/builtin/branch.c
    +++ b/builtin/branch.c
    @@ -7,4 +7,2 @@

    -#include "cache.h"
    -#include "config.h"
     #include "color.h"
    diff --git a/cache.h b/cache.h
    index ca36b44ee0..d4146dbf8a 100644
    --- a/cache.h
    +++ b/cache.h
    @@ -3,2 +3,4 @@

    +#include "cache.h"
    +#include "config.h"
     #include "git-compat-util.h"

So is there some "must be at least two consecutive lines" condition for
not-plain, or is something else going on here?