Update showindex example

I find the showindex example to be pretty useful on occasion, but
there were are couple of output tweaks I wanted, plus I wanted the
ability to specify a path to an actual index file instead of having
to open the whole repository.  This makes those changes and expands
the example slightly.

examples/showindex.c

 #include <git2.h>
 #include <stdio.h>
+#include <string.h>
 
 int main (int argc, char** argv)
 {
-	git_repository *repo;
+	git_repository *repo = NULL;
 	git_index *index;
 	unsigned int i, ecount;
 	char *dir = ".";
+	size_t dirlen;
 	char out[41];
 	out[40] = '\0';
 
 	if (argc > 1)
 		dir = argv[1];
-	if (argc > 2) {
+	if (!dir || argc > 2) {
 		fprintf(stderr, "usage: showindex [<repo-dir>]\n");
 		return 1;
 	}
 
-	if (git_repository_open_ext(&repo, dir, 0, NULL) < 0) {
-		fprintf(stderr, "could not open repository: %s\n", dir);
-		return 1;
+	dirlen = strlen(dir);
+	if (dirlen > 5 && strcmp(dir + dirlen - 5, "index") == 0) {
+		if (git_index_open(&index, dir) < 0) {
+			fprintf(stderr, "could not open index: %s\n", dir);
+			return 1;
+		}
+	} else {
+		if (git_repository_open_ext(&repo, dir, 0, NULL) < 0) {
+			fprintf(stderr, "could not open repository: %s\n", dir);
+			return 1;
+		}
+		if (git_repository_index(&index, repo) < 0) {
+			fprintf(stderr, "could not open repository index\n");
+			return 1;
+		}
 	}
 
-	git_repository_index(&index, repo);
 	git_index_read(index);
 
 	ecount = git_index_entrycount(index);
@@ -37,11 +50,10 @@ int main (int argc, char** argv)
 		printf("File Path: %s\n", e->path);
 		printf("    Stage: %d\n", git_index_entry_stage(e));
 		printf(" Blob SHA: %s\n", out);
-		printf("File Size: %d\n", (int)e->file_size);
-		printf("   Device: %d\n", (int)e->dev);
-		printf("    Inode: %d\n", (int)e->ino);
-		printf("      UID: %d\n", (int)e->uid);
-		printf("      GID: %d\n", (int)e->gid);
+		printf("File Mode: %07o\n", e->mode);
+		printf("File Size: %d bytes\n", (int)e->file_size);
+		printf("Dev/Inode: %d/%d\n", (int)e->dev, (int)e->ino);
+		printf("  UID/GID: %d/%d\n", (int)e->uid, (int)e->gid);
 		printf("    ctime: %d\n", (int)e->ctime.seconds);
 		printf("    mtime: %d\n", (int)e->mtime.seconds);
 		printf("\n");