Re-instate index file write optimization

This makes "git-update-index" avoid the new index file write if it didn't 
make any changes to the index.

It still doesn't make things like "git status" be read-only operations in 
general, but if the index file doesn't need refreshing, it now will at 
least avoid making unnecessary changes.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
---

This only makes sense after applying my previous patch, since otherwise a 
"--refresh" will always end up marking the index dirty anyway.

Also, while I tried to verify that we always mark the index dirty when we 
change it, I can't guarantee it. In particular, since we now map the index 
file with PROT_READ | PROT_WRITE, somebody can modify the index entries in 
place without getting a SIGSEGV.

Originally you couldn't do the modify-in-place, and used to be that you 
could depend on the active_cache[] being unmodified only by actually 
assigning to the pointer array itself.

I don't think git-update-index writes to the entries directly, though 
(git-read-tree does, but it doesn't do the optimization).

diff --git a/update-index.c b/update-index.c
--- a/update-index.c
+++ b/update-index.c

@@ -391,9 +391,11 @@ int main(int argc, const char **argv)
 			update_one(buf.buf, prefix, prefix_length);
 		}
 	}
-	if (write_cache(newfd, active_cache, active_nr) ||
-	    commit_index_file(&cache_file))
-		die("Unable to write new cachefile");
+	if (active_cache_changed) {
+		if (write_cache(newfd, active_cache, active_nr) ||
+		    commit_index_file(&cache_file))
+			die("Unable to write new cachefile");
+	}
 
 	return has_errors ? 1 : 0;
 }