multi-pack-index: use --object-dir real path

The --object-dir argument to 'git multi-pack-index' allows a user to
specify an alternate to use instead of the local $GITDIR. This is used
by third-party tools like VFS for Git to maintain the pack-files in a
"shared object cache" used by multiple clones.

On Windows, the user can specify a path using a Windows-style file path
with backslashes such as "C:\Path\To\ObjectDir". This same path style is
used in the .git/objects/info/alternates file, so it already matches the
path of that alternate. However, find_odb() converts these paths to
real-paths for the comparison, which use forward slashes. As of the
previous change, lookup_multi_pack_index() uses real-paths, so it
correctly finds the target multi-pack-index when given these paths.

Some commands such as 'git multi-pack-index repack' call child processes
using the object_dir value, so it can be helpful to convert the path to
the real-path before sending it to those locations.

Adding the normalization in builtin/multi-pack-index.c is a little
complicated because of how the sub-commands were split in 60ca947
(builtin/multi-pack-index.c: split sub-commands, 2021-03-30). The
--object-dir argument could be parsed before the sub-command name _or_
after it. Thus, create a normalize_object_dir() helper to call after all
arguments are parsed, but before any logic is run on that object dir.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>

builtin/multi-pack-index.c

@@ -90,6 +90,14 @@ static void read_packs_from_stdin(struct string_list *to)
 	strbuf_release(&buf);
 }
 
+static void normalize_object_dir(void)
+{
+	if (!opts.object_dir)
+		opts.object_dir = get_object_directory();
+	else
+		opts.object_dir = real_pathdup(opts.object_dir, 1);
+}
+
 static int cmd_multi_pack_index_write(int argc, const char **argv)
 {
 	struct option *options;
@@ -127,6 +135,8 @@ static int cmd_multi_pack_index_write(int argc, const char **argv)
 
 	FREE_AND_NULL(options);
 
+	normalize_object_dir();
+
 	if (opts.stdin_packs) {
 		struct string_list packs = STRING_LIST_INIT_DUP;
 		int ret;
@@ -169,6 +179,8 @@ static int cmd_multi_pack_index_verify(int argc, const char **argv)
 
 	FREE_AND_NULL(options);
 
+	normalize_object_dir();
+
 	return verify_midx_file(the_repository, opts.object_dir, opts.flags);
 }
 
@@ -195,6 +207,8 @@ static int cmd_multi_pack_index_expire(int argc, const char **argv)
 
 	FREE_AND_NULL(options);
 
+	normalize_object_dir();
+
 	return expire_midx_packs(the_repository, opts.object_dir, opts.flags);
 }
 
@@ -225,6 +239,8 @@ static int cmd_multi_pack_index_repack(int argc, const char **argv)
 
 	FREE_AND_NULL(options);
 
+	normalize_object_dir();
+
 	return midx_repack(the_repository, opts.object_dir,
 			   (size_t)opts.batch_size, opts.flags);
 }
@@ -241,9 +257,6 @@ int cmd_multi_pack_index(int argc, const char **argv,
 			     builtin_multi_pack_index_usage,
 			     PARSE_OPT_STOP_AT_NON_OPTION);
 
-	if (!opts.object_dir)
-		opts.object_dir = get_object_directory();
-
 	if (!argc)
 		goto usage;