Teach --dirstat not to completely ignore rearranged lines within a file

Currently, the --dirstat analysis ignores when lines within a file are
rearranged, because the "damage" calculated by show_dirstat() is 0.
However, if the object name has changed, we already know that there is
some damage, and it is unintuitive to claim there is _no_ damage.

Teach show_dirstat() to assign a minimum amount of damage (== 1) to
entries for which the analysis otherwise yields zero damage, to still
represent that these files are changed, instead of saying that there
is no change.

Also, skip --dirstat analysis when the object names are the same (e.g. for
a pure file rename).

Signed-off-by: Johan Herland <johan@herland.net>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4013-diff-various.sh

@@ -300,9 +300,7 @@ diff --no-index --name-status -- dir2 dir
 diff --no-index dir dir3
 diff master master^ side
 diff --dirstat master~1 master~2
-# --dirstat doesn't notice changes that simply rearrange existing lines
 diff --dirstat initial rearrange
-# ...but --dirstat-by-file does notice changes that only rearrange lines
 diff --dirstat-by-file initial rearrange
 EOF
 

t/t4013/diff.diff_--dirstat_initial_rearrange

@@ -1,2 +1,3 @@
 $ git diff --dirstat initial rearrange
+ 100.0% dir/
 $