update-index: add --show-index-version

"git update-index --index-version N" is used to set the index format
version to a specific version, but there was no way to query the
current version used in the on-disk index file.

Teach the command a new "--show-index-version" option, and also
teach the "--index-version N" option to report what the version was
when run with the "--verbose" option.

Helped-by: Linus Arver <linusa@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-update-index.txt

@@ -162,7 +162,8 @@ you will need to handle the situation manually.
 	Write the resulting index out in the named on-disk format version.
 	Supported versions are 2, 3 and 4. The current default version is 2
 	or 3, depending on whether extra features are used, such as
-	`git add -N`.
+	`git add -N`.  With `--verbose`, also report the version the index
+	file uses before and after this command.
 +
 Version 4 performs a simple pathname compression that reduces index
 size by 30%-50% on large repositories, which results in faster load
@@ -171,6 +172,9 @@ and support for it was added to libgit2 in 2016 and to JGit in 2020.
 Older versions of this manual page called it "relatively young", but
 it should be considered mature technology these days.
 
+--show-index-version::
+	Report the index format version used by the on-disk index file.
+	See `--index-version` above.
 
 -z::
 	Only meaningful with `--stdin` or `--index-info`; paths are

builtin/update-index.c

@@ -1089,6 +1089,8 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 			resolve_undo_clear_callback),
 		OPT_INTEGER(0, "index-version", &preferred_index_format,
 			N_("write index in this format")),
+		OPT_SET_INT(0, "show-index-version", &preferred_index_format,
+			    N_("report on-disk index format version"), -1),
 		OPT_BOOL(0, "split-index", &split_index,
 			N_("enable or disable split index")),
 		OPT_BOOL(0, "untracked-cache", &untracked_cache,
@@ -1181,15 +1183,20 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 
 	getline_fn = nul_term_line ? strbuf_getline_nul : strbuf_getline_lf;
 	if (preferred_index_format) {
-		if (preferred_index_format < INDEX_FORMAT_LB ||
+		if (preferred_index_format < 0) {
-		    INDEX_FORMAT_UB < preferred_index_format)
+			printf(_("%d\n"), the_index.version);
+		} else if (preferred_index_format < INDEX_FORMAT_LB ||
+			   INDEX_FORMAT_UB < preferred_index_format) {
 			die("index-version %d not in range: %d..%d",
 			    preferred_index_format,
 			    INDEX_FORMAT_LB, INDEX_FORMAT_UB);
-
+		} else {
-		if (the_index.version != preferred_index_format)
+			if (the_index.version != preferred_index_format)
-			the_index.cache_changed |= SOMETHING_CHANGED;
+				the_index.cache_changed |= SOMETHING_CHANGED;
-		the_index.version = preferred_index_format;
+			report(_("index-version: was %d, set to %d"),
+			       the_index.version, preferred_index_format);
+			the_index.version = preferred_index_format;
+		}
 	}
 
 	if (read_from_stdin) {

t/t2107-update-index-basic.sh

@@ -111,4 +111,35 @@ test_expect_success '--chmod=+x and chmod=-x in the same argument list' '
 	test_cmp expect actual
 '
 
+test_expect_success '--index-version' '
+	git commit --allow-empty -m snap &&
+	git reset --hard &&
+	git rm -f -r --cached . &&
+
+	# The default index version is 2 --- update this test
+	# when you change it in the code
+	git update-index --show-index-version >actual &&
+	echo 2 >expect &&
+	test_cmp expect actual &&
+
+	# The next test wants us to be using version 2
+	git update-index --index-version 2 &&
+
+	git update-index --index-version 4 --verbose >actual &&
+	echo "index-version: was 2, set to 4" >expect &&
+	test_cmp expect actual &&
+
+	git update-index --index-version 4 --verbose >actual &&
+	echo "index-version: was 4, set to 4" >expect &&
+	test_cmp expect actual &&
+
+	git update-index --index-version 2 --verbose >actual &&
+	echo "index-version: was 4, set to 2" >expect &&
+	test_cmp expect actual &&
+
+	# non-verbose should be silent
+	git update-index --index-version 4 >actual &&
+	test_must_be_empty actual
+'
+
 test_done