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>

Documentation/diff-options.txt

@@ -74,8 +74,8 @@ endif::git-format-patch[]
 	counted for the parent directory, unless `--cumulative` is used.
 +
 Note that the `--dirstat` option computes the changes while ignoring
-pure code movements within a file.  In other words, rearranging lines
+the amount of pure code movements within a file.  In other words,
-in a file is not counted as a change.
+rearranging lines in a file is not counted as much as other changes.
 
 --dirstat-by-file[=<limit>]::
 	Same as `--dirstat`, but counts changed files instead of lines.

diff.c

@@ -1548,6 +1548,16 @@ static void show_dirstat(struct diff_options *options)
 		else
 			content_changed = 1;
 
+		if (!content_changed) {
+			/*
+			 * The SHA1 has not changed, so pre-/post-content is
+			 * identical. We can therefore skip looking at the
+			 * file contents altogether.
+			 */
+			damage = 0;
+			goto found_damage;
+		}
+
 		if (DIFF_OPT_TST(options, DIRSTAT_BY_FILE)) {
 			/*
 			 * In --dirstat-by-file mode, we don't really need to
@@ -1556,7 +1566,7 @@ static void show_dirstat(struct diff_options *options)
 			 * add this file to the list of results
 			 * (with each file contributing equal damage).
 			 */
-			damage = content_changed ? 1 : 0;
+			damage = 1;
 			goto found_damage;
 		}
 
@@ -1583,8 +1593,15 @@ static void show_dirstat(struct diff_options *options)
 		 * Original minus copied is the removed material,
 		 * added is the new material.  They are both damages
 		 * made to the preimage.
+		 * If the resulting damage is zero, we know that
+		 * diffcore_count_changes() considers the two entries to
+		 * be identical, but since content_changed is true, we
+		 * know that there must have been _some_ kind of change,
+		 * so we force all entries to have damage > 0.
 		 */
 		damage = (p->one->size - copied) + added;
+		if (!damage)
+			damage = 1;
 
 found_damage:
 		ALLOC_GROW(dir.files, dir.nr + 1, dir.alloc);

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/
 $