rehabilitate 'git index-pack' inside the object store

Before commit d0b92a3 it was possible to run 'git index-pack'
directly in the .git/objects/pack/ directory.  Restore that ability.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

index-pack.c

@@ -879,10 +879,26 @@ int main(int argc, char **argv)
 	char *index_name_buf = NULL, *keep_name_buf = NULL;
 	struct pack_idx_entry **idx_objects;
 	unsigned char pack_sha1[20];
-	int nongit = 0;
 
-	setup_git_directory_gently(&nongit);
-	git_config(git_index_pack_config, NULL);
+	/*
+	 * We wish to read the repository's config file if any, and
+	 * for that it is necessary to call setup_git_directory_gently().
+	 * However if the cwd was inside .git/objects/pack/ then we need
+	 * to go back there or all the pack name arguments will be wrong.
+	 * And in that case we cannot rely on any prefix returned by
+	 * setup_git_directory_gently() either.
+	 */
+	{
+		char cwd[PATH_MAX+1];
+		int nongit;
+
+		if (!getcwd(cwd, sizeof(cwd)-1))
+			die("Unable to get current working directory");
+		setup_git_directory_gently(&nongit);
+		git_config(git_index_pack_config, NULL);
+		if (chdir(cwd))
+			die("Cannot come back to cwd");
+	}
 
 	for (i = 1; i < argc; i++) {
 		char *arg = argv[i];

t/t5302-pack-index.sh

@@ -177,4 +177,14 @@ test_expect_success \
        ".git/objects/pack/pack-${pack1}.pack" 2>&1) &&
      echo "$err" | grep "CRC mismatch"'
 
+test_expect_success 'running index-pack in the object store' '
+    rm -f .git/objects/pack/* &&
+    cp test-1-${pack1}.pack .git/objects/pack/pack-${pack1}.pack &&
+    (
+	cd .git/objects/pack
+	git index-pack pack-${pack1}.pack
+    ) &&
+    test -f .git/objects/pack/pack-${pack1}.idx
+'
+
 test_done