archive: initialize archivers earlier

Initialize archivers as soon as possible when running git-archive.
Various non-obvious behavior depends on having the archivers
initialized, such as determining the desired archival format from the
provided filename.

Since 08716b3c11 ("archive: refactor file extension format-guessing",
2011-06-21), archive_format_from_filename() has used the registered
archivers to match filenames (provided via --output) to archival
formats. However, when git-archive is executed with --remote, format
detection happens before the archivers have been registered. This causes
archives from remotes to always be generated as TAR files, regardless of
the actual filename (unless an explicit --format is provided).

This patch fixes that behavior; archival format is determined properly
from the output filename, even when --remote is used.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

archive.h

@@ -43,6 +43,7 @@ extern void register_archiver(struct archiver *);
 
 extern void init_tar_archiver(void);
 extern void init_zip_archiver(void);
+extern void init_archivers(void);
 
 typedef int (*write_archive_entry_fn_t)(struct archiver_args *args,
 					const struct object_id *oid,