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: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

cache.h

@@ -179,6 +179,7 @@ extern int commit_index_file(struct cache_file *);
 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

config.c

@@ -214,6 +214,11 @@ int git_default_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.symrefsonly")) {
+		only_use_symrefs = git_config_bool(var, value);
+		return 0;
+	}
+
 	if (!strcmp(var, "user.name")) {
 		strncpy(git_default_name, value, sizeof(git_default_name));
 		return 0;

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;

refs.c

@@ -121,10 +121,12 @@ int create_symref(const char *git_HEAD, const char *refs_heads_master)
 	int fd, len, written;
 
 #if USE_SYMLINK_HEAD
-	unlink(git_HEAD);
-	if (!symlink(refs_heads_master, git_HEAD))
-		return 0;
-	fprintf(stderr, "no symlink - falling back to symbolic ref\n");
+	if (!only_use_symrefs) {
+		unlink(git_HEAD);
+		if (!symlink(refs_heads_master, git_HEAD))
+			return 0;
+		fprintf(stderr, "no symlink - falling back to symbolic ref\n");
+	}
 #endif
 
 	len = snprintf(ref, sizeof(ref), "ref: %s\n", refs_heads_master);

symbolic-ref.c

@@ -20,6 +20,7 @@ static void check_symref(const char *HEAD)
 int main(int argc, const char **argv)
 {
 	setup_git_directory();
+	git_config(git_default_config);
 	switch (argc) {
 	case 2:
 		check_symref(argv[1]);