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>

environment.c

@@ -12,6 +12,7 @@
 char git_default_email[MAX_GITNAME];
 char git_default_name[MAX_GITNAME];
 int trust_executable_bit = 1;
+int only_use_symrefs = 0;
 
 static char *git_dir, *git_object_dir, *git_index_file, *git_refs_dir,
 	*git_graft_file;