Log ref updates to logs/refs/<ref>

If config parameter core.logAllRefUpdates is true or the log
file already exists then append a line to ".git/logs/refs/<ref>"
whenever git-update-ref <ref> is executed.  Each log line contains
the following information:

  oldsha1 <SP> newsha1 <SP> committer <LF>

where committer is the current user, date, time and timezone in
the standard GIT ident format.  If the caller is unable to append
to the log file then git-update-ref will fail without updating <ref>.

An optional message may be included in the log line with the -m flag.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

config.c

@@ -269,6 +269,11 @@ int git_default_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.logallrefupdates")) {
+		log_all_ref_updates = git_config_bool(var, value);
+		return 0;
+	}
+
 	if (!strcmp(var, "core.warnambiguousrefs")) {
 		warn_ambiguous_refs = git_config_bool(var, value);
 		return 0;