--dirstat: Describe non-obvious differences relative to --stat or regular diff

Also add a testcase documenting the current behavior.

Improved-by: Junio C Hamano <gitster@pobox.com>
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

@@ -72,6 +72,10 @@ endif::git-format-patch[]
 	a cut-off percent (3% by default) are not shown. The cut-off percent
 	can be set with `--dirstat=<limit>`. Changes in a child directory are not
 	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
+in a file is not counted as a change.
 
 --dirstat-by-file[=<limit>]::
 	Same as `--dirstat`, but counts changed files instead of lines.

t/t4013-diff-various.sh

@@ -80,18 +80,31 @@ test_expect_success setup '
 
 	git config log.showroot false &&
 	git commit --amend &&
+
+	GIT_AUTHOR_DATE="2006-06-26 00:06:00 +0000" &&
+	GIT_COMMITTER_DATE="2006-06-26 00:06:00 +0000" &&
+	export GIT_AUTHOR_DATE GIT_COMMITTER_DATE &&
+	git checkout -b rearrange initial &&
+	for i in B A; do echo $i; done >dir/sub &&
+	git add dir/sub &&
+	git commit -m "Rearranged lines in dir/sub" &&
+	git checkout master &&
+
 	git show-branch
 '
 
 : <<\EOF
 ! [initial] Initial
  * [master] Merge branch 'side'
-  ! [side] Side
+  ! [rearrange] Rearranged lines in dir/sub
----
+   ! [side] Side
- -  [master] Merge branch 'side'
+----
- *+ [side] Side
+  +  [rearrange] Rearranged lines in dir/sub
- *  [master^] Second
+ -   [master] Merge branch 'side'
-+*+ [initial] Initial
+ * + [side] Side
+ *   [master^] Third
+ *   [master~2] Second
++*++ [initial] Initial
 EOF
 
 V=`git version | sed -e 's/^git version //' -e 's/\./\\./g'`
@@ -287,6 +300,8 @@ 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
 EOF
 
 test_expect_success 'log -S requires an argument' '

t/t4013/diff.diff_--dirstat_initial_rearrange

@@ -0,0 +1,2 @@
+$ git diff --dirstat initial rearrange
+$

t/t4013/diff.format-patch_--stdout_--cover-letter_-n_initial..master^

@@ -1,7 +1,7 @@
 $ git format-patch --stdout --cover-letter -n initial..master^
 From 9a6d4949b6b76956d9d5e26f2791ec2ceff5fdc0 Mon Sep 17 00:00:00 2001
 From: C O Mitter <committer@example.com>
-Date: Mon, 26 Jun 2006 00:05:00 +0000
+Date: Mon, 26 Jun 2006 00:06:00 +0000
 Subject: [DIFFERENT_PREFIX 0/2] *** SUBJECT HERE ***
 
 *** BLURB HERE ***

t/t4013/diff.log_--decorate=full_--all

@@ -1,4 +1,10 @@
 $ git log --decorate=full --all
+commit cd4e72fd96faed3f0ba949dc42967430374e2290 (refs/heads/rearrange)
+Author: A U Thor <author@example.com>
+Date:   Mon Jun 26 00:06:00 2006 +0000
+
+    Rearranged lines in dir/sub
+
 commit 59d314ad6f356dd08601a4cd5e530381da3e3c64 (HEAD, refs/heads/master)
 Merge: 9a6d494 c7a2ab9
 Author: A U Thor <author@example.com>

t/t4013/diff.log_--decorate_--all

@@ -1,4 +1,10 @@
 $ git log --decorate --all
+commit cd4e72fd96faed3f0ba949dc42967430374e2290 (rearrange)
+Author: A U Thor <author@example.com>
+Date:   Mon Jun 26 00:06:00 2006 +0000
+
+    Rearranged lines in dir/sub
+
 commit 59d314ad6f356dd08601a4cd5e530381da3e3c64 (HEAD, master)
 Merge: 9a6d494 c7a2ab9
 Author: A U Thor <author@example.com>