Add config variable core.symrefsonly

This allows you to force git to avoid symlinks for refs. Just add
something like

	[core]
		symrefsonly = true

to .git/config.

Don“t forget to "git checkout your_branch", or it does not do anything...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/cache.h b/cache.h
index 677c6ac..9a6bfb9 100644
--- a/cache.h
+++ b/cache.h
@@ -179,6 +179,7 @@
 extern void rollback_index_file(struct cache_file *);
 
 extern int trust_executable_bit;
+extern int only_use_symrefs;
 
 #define MTIME_CHANGED	0x0001
 #define CTIME_CHANGED	0x0002