Make git log --follow find copies among unmodified files.

'git log --follow <path>' don't track copies from unmodified
files, and this patch fix it.

Signed-off-by: Bo Yang <struggleyb.nku@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-log.txt

@@ -56,7 +56,7 @@ include::diff-options.txt[]
 	commits, and doesn't limit diff for those commits.
 
 --follow::
-	Continue listing the history of a file beyond renames.
+	Continue listing the history of a file beyond renames/copies.
 
 --log-size::
 	Before the log message print out its size in bytes. Intended

t/t4205-log-follow-harder-copies.sh

@@ -0,0 +1,56 @@
+#!/bin/sh
+#
+# Copyright (c) 2010 Bo Yang
+#
+
+test_description='Test --follow should always find copies hard in git log.
+
+'
+. ./test-lib.sh
+. "$TEST_DIRECTORY"/diff-lib.sh
+
+echo >path0 'Line 1
+Line 2
+Line 3
+'
+
+test_expect_success \
+    'add a file path0 and commit.' \
+    'git add path0 &&
+     git commit -m "Add path0"'
+
+echo >path0 'New line 1
+New line 2
+New line 3
+'
+test_expect_success \
+    'Change path0.' \
+    'git add path0 &&
+     git commit -m "Change path0"'
+
+cat <path0 >path1
+test_expect_success \
+    'copy path0 to path1.' \
+    'git add path1 &&
+     git commit -m "Copy path1 from path0"'
+
+test_expect_success \
+    'find the copy path0 -> path1 harder' \
+    'git log --follow --name-status --pretty="format:%s"  path1 > current'
+
+cat >expected <<\EOF
+Copy path1 from path0
+C100	path0	path1
+
+Change path0
+M	path0
+
+Add path0
+A	path0
+EOF
+
+test_expect_success \
+    'validate the output.' \
+    'compare_diff_patch current expected'
+
+test_done

tree-diff.c

@@ -346,7 +346,7 @@ static void try_to_follow_renames(struct tree_desc *t1, struct tree_desc *t2, co
 
 	diff_setup(&diff_opts);
 	DIFF_OPT_SET(&diff_opts, RECURSIVE);
-	diff_opts.detect_rename = DIFF_DETECT_RENAME;
+	DIFF_OPT_SET(&diff_opts, FIND_COPIES_HARDER);
 	diff_opts.output_format = DIFF_FORMAT_NO_OUTPUT;
 	diff_opts.single_follow = opt->paths[0];
 	diff_opts.break_opt = opt->break_opt;