range-diff: show submodule changes irrespective of diff.submodule

After generating diffs for each range to be compared using a 'git log'
invocation, range-diff.c::read_patches looks for the "diff --git" header
in those diffs to recognize the beginning of a new change.

In a project with submodules, and with 'diff.submodule=log' set in the
config, this header is missing for the diff of a changed submodule, so
any submodule changes are quietly ignored in the range-diff.

When 'diff.submodule=diff' is set in the config, the "diff --git" header
is also missing for the submodule itself, but is shown for submodule
content changes, which can easily confuse 'git range-diff' and lead to
errors such as:

    error: git apply: bad git-diff - inconsistent old filename on line 1
    error: could not parse git header 'diff --git path/to/submodule/and/some/file/within
    '
    error: could not parse log for '@{u}..@{1}'

Force the submodule diff format to its default ("short") when invoking
'git log' to generate the patches for each range, such that submodule
changes are always detected.

Add a test, including an invocation with '--creation-factor=100' to
force the second commit in the range not to be considered a complete
rewrite, in order to verify we do indeed get the "short" format.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

range-diff.c

@@ -44,7 +44,7 @@ static int read_patches(const char *range, struct string_list *list,
 
 	strvec_pushl(&cp.args, "log", "--no-color", "-p", "--no-merges",
 		     "--reverse", "--date-order", "--decorate=no",
-		     "--no-prefix",
+		     "--no-prefix", "--submodule=short",
 		     /*
 		      * Choose indicators that are not used anywhere
 		      * else in diffs, but still look reasonable

t/t3206-range-diff.sh

@@ -772,4 +772,55 @@ test_expect_success '--left-only/--right-only' '
 	test_cmp expect actual
 '
 
+test_expect_success 'submodule changes are shown irrespective of diff.submodule' '
+	git init sub-repo &&
+	test_commit -C sub-repo sub-first &&
+	sub_oid1=$(git -C sub-repo rev-parse HEAD) &&
+	test_commit -C sub-repo sub-second &&
+	sub_oid2=$(git -C sub-repo rev-parse HEAD) &&
+	test_commit -C sub-repo sub-third &&
+	sub_oid3=$(git -C sub-repo rev-parse HEAD) &&
+
+	git checkout -b main-sub topic &&
+	git submodule add ./sub-repo sub &&
+	git -C sub checkout --detach sub-first &&
+	git commit -m "add sub" sub &&
+	sup_oid1=$(git rev-parse --short HEAD) &&
+	git checkout -b topic-sub &&
+	git -C sub checkout sub-second &&
+	git commit -m "change sub" sub &&
+	sup_oid2=$(git rev-parse --short HEAD) &&
+	git checkout -b modified-sub main-sub &&
+	git -C sub checkout sub-third &&
+	git commit -m "change sub" sub &&
+	sup_oid3=$(git rev-parse --short HEAD) &&
+	sup_oid0=$(test_oid __) &&
+
+	test_config diff.submodule log &&
+	git range-diff topic topic-sub modified-sub >actual &&
+	cat >expect <<-EOF &&
+	1:  $sup_oid1 = 1:  $sup_oid1 add sub
+	2:  $sup_oid2 < -:  $sup_oid0 change sub
+	-:  $sup_oid0 > 2:  $sup_oid3 change sub
+	EOF
+	test_cmp expect actual &&
+	test_config diff.submodule diff &&
+	git range-diff topic topic-sub modified-sub >actual &&
+	git range-diff --creation-factor=100 topic topic-sub modified-sub >actual &&
+	cat >expect <<-EOF &&
+	1:  $sup_oid1 = 1:  $sup_oid1 add sub
+	2:  $sup_oid2 ! 2:  $sup_oid3 change sub
+	    @@ Commit message
+	      ## sub ##
+	     @@
+	     -Subproject commit $sub_oid1
+	    -+Subproject commit $sub_oid2
+	    ++Subproject commit $sub_oid3
+	EOF
+	test_cmp expect actual &&
+	test_config diff.submodule diff &&
+	git range-diff --creation-factor=100 topic topic-sub modified-sub >actual &&
+	test_cmp expect actual
+'
+
 test_done