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>
diff --git a/builtin/upload-archive.c b/builtin/upload-archive.c
index 25d9116..0188797 100644
--- a/builtin/upload-archive.c
+++ b/builtin/upload-archive.c
@@ -28,6 +28,8 @@ int cmd_upload_archive_writer(int argc, const char **argv, const char *prefix)
 	if (!enter_repo(argv[1], 0))
 		die("'%s' does not appear to be a git repository", argv[1]);
 
+	init_archivers();
+
 	/* put received options in sent_argv[] */
 	argv_array_push(&sent_argv, "git-upload-archive");
 	for (;;) {