t5300: confirm failure of git index-pack when non-idx suffix requested

Add test case to demonstrate that `git index-pack -o <idx-path>
pack-path` fails if <idx-path> does not end in ".idx" when `--rev-index`
is enabled.

In e37d0b8 (builtin/index-pack.c: write reverse indexes, 2021-01-25)
we learned to create `.rev` reverse indexes in addition to `.idx` index
files.  The `.rev` file pathname is constructed by replacing the suffix
on the `.idx` file.  The code assumes a hard-coded "idx" suffix.

In a8dd7e0 (config: enable `pack.writeReverseIndex` by default,
2023-04-12) reverse indexes were enabled by default.

If the `-o <idx-path>` argument is used, the index file may have a
different suffix.  This causes an error when it tries to create the
reverse index pathname.

The test here demonstrates the failure.  (The test forces `--rev-index`
to avoid interaction with `GIT_TEST_NO_WRITE_REV_INDEX` during CI runs.)

Signed-off-by: Jeff Hostetler <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

t/t5300-pack-object.sh

@@ -355,6 +355,31 @@ test_expect_success 'build pack index for an existing pack' '
 	:
 '
 
+# The `--rev-index` option of `git index-pack` is now the default, so
+# a `foo.rev` REV file will be created when a `foo.idx` IDX file is
+# created.  Normally, these pathnames are based upon the `foo.pack`
+# PACK file pathname.
+#
+# However, the `-o` option lets you set the pathname of the IDX file
+# indepdent of the PACK file.
+#
+# Verify what happens if these suffixes are changed.
+#
+test_expect_success 'complain about index name' '
+	# Normal case { .pack, .idx, .rev }
+	cat test-1-${packname_1}.pack >test-complain-0.pack &&
+	git index-pack -o test-complain-0.idx --rev-index test-complain-0.pack &&
+	test -f test-complain-0.idx &&
+	test -f test-complain-0.rev &&
+
+	# Non .idx suffix
+	cat test-1-${packname_1}.pack >test-complain-1.pack &&
+	test_must_fail git index-pack -o test-complain-1.idx-suffix --rev-index test-complain-1.pack 2>err &&
+	grep "does not end" err &&
+	! test -f test-complain-1.idx-suffix &&
+	! test -f test-complain-1.rev
+'
+
 test_expect_success 'unpacking with --strict' '
 
 	for j in a b c d e f g