Add a "git-show-index" helper that shows the contents of a pack index

This was invaluable for debugging the zero-sized compression issue, and
might be useful for scripting too, if people want to see the contents of
a pack.

Makefile

@@ -43,7 +43,8 @@ PROG=   git-update-cache git-diff-files git-init-db git-write-tree \
 	git-get-tar-commit-id git-apply git-stripspace \
 	git-diff-stages git-rev-parse git-patch-id git-pack-objects \
 	git-unpack-objects git-verify-pack git-receive-pack git-send-pack \
-	git-prune-packed git-fetch-pack git-upload-pack git-clone-pack
+	git-prune-packed git-fetch-pack git-upload-pack git-clone-pack \
+	git-show-index
 
 all: $(PROG)
 

show-index.c

@@ -0,0 +1,28 @@
+#include "cache.h"
+
+int main(int argc, char **argv)
+{
+	int i;
+	unsigned nr;
+	unsigned int entry[6];
+	static unsigned int top_index[256];
+
+	if (fread(top_index, sizeof(top_index), 1, stdin) != 1)
+		die("unable to read idex");
+	nr = 0;
+	for (i = 0; i < 256; i++) {
+		unsigned n = ntohl(top_index[i]);
+		if (n < nr)
+			die("corrupt index file");
+		nr = n;
+	}
+	for (i = 0; i < nr; i++) {
+		unsigned offset;
+
+		if (fread(entry, 24, 1, stdin) != 1)
+			die("unable to read entry %u/%u", i, nr);
+		offset = ntohl(entry[0]);
+		printf("%u %s\n", offset, sha1_to_hex((void *)(entry+1)));
+	}
+	return 0;
+}