Don't care about st_dev in the index file

Thomas Glanzmann points out that it doesn't work well with different
clients accessing the repository over NFS - they have different views
on what the "device" for the filesystem is.

Of course, other filesystems may not even have stable inode numbers.
But we don't care. At least for now.

Makefile

@@ -2,7 +2,9 @@
 # 1461501637330902918203684832716283019655932542976 hashes do not give you
 # enough guarantees about no collisions between objects ever hapenning.
 #
-# -DNSEC if you want git to care about sub-second file mtimes and ctimes.
+# -DUSE_NSEC if you want git to care about sub-second file mtimes and ctimes.
+# -DUSE_STDEV if you want git to care about st_dev changing
+#
 # Note that you need some new glibc (at least >2.2.4) for this, and it will
 # BREAK YOUR LOCAL DIFFS! show-diff and anything using it will likely randomly
 # break unless your underlying filesystem supports those sub-second times

read-cache.c

@@ -17,7 +17,7 @@ void fill_stat_cache_info(struct cache_entry *ce, struct stat *st)
 {
 	ce->ce_ctime.sec = htonl(st->st_ctime);
 	ce->ce_mtime.sec = htonl(st->st_mtime);
-#ifdef NSEC
+#ifdef USE_NSEC
 	ce->ce_ctime.nsec = htonl(st->st_ctim.tv_nsec);
 	ce->ce_mtime.nsec = htonl(st->st_mtim.tv_nsec);
 #endif
@@ -50,7 +50,7 @@ int ce_match_stat(struct cache_entry *ce, struct stat *st)
 	if (ce->ce_ctime.sec != htonl(st->st_ctime))
 		changed |= CTIME_CHANGED;
 
-#ifdef NSEC
+#ifdef USE_NSEC
 	/*
 	 * nsec seems unreliable - not all filesystems support it, so
 	 * as long as it is in the inode cache you get right nsec
@@ -65,9 +65,19 @@ int ce_match_stat(struct cache_entry *ce, struct stat *st)
 	if (ce->ce_uid != htonl(st->st_uid) ||
 	    ce->ce_gid != htonl(st->st_gid))
 		changed |= OWNER_CHANGED;
-	if (ce->ce_dev != htonl(st->st_dev) ||
-	    ce->ce_ino != htonl(st->st_ino))
+	if (ce->ce_ino != htonl(st->st_ino))
 		changed |= INODE_CHANGED;
+
+#ifdef USE_STDEV
+	/*
+	 * st_dev breaks on network filesystems where different
+	 * clients will have different views of what "device"
+	 * the filesystem is on
+	 */
+	if (ce->ce_dev != htonl(st->st_dev))
+		changed |= INODE_CHANGED;
+#endif
+
 	if (ce->ce_size != htonl(st->st_size))
 		changed |= DATA_CHANGED;
 	return changed;