core.logallrefupdates create new log file only for branch heads.

It used to mean "create log file for any ref that is updated",
but now it creates new log files only for branch heads.

The old behaviour made this configuration less useful than
otherwise it would be; automatically creating log file for tags
is almost always not useful.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/config.txt

@@ -71,12 +71,16 @@ core.preferSymlinkRefs::
 	expect HEAD to be a symbolic link.
 
 core.logAllRefUpdates::
-	If true, `git-update-ref` will append a line to
-	"$GIT_DIR/logs/<ref>" listing the new SHA1 and the date/time
-	of the update.	If the file does not exist it will be
-	created automatically.	This information can be used to
-	determine what commit was the tip of a branch "2 days ago".
-	This value is false by default (no logging).
+	Updates to a ref <ref> is logged to the file
+	"$GIT_DIR/logs/<ref>", by appending the new and old
+	SHA1, the date/time and the reason of the update, but
+	only when the file exists.  If this configuration
+	variable is set to true, missing "$GIT_DIR/logs/<ref>"
+	file is automatically created for branch heads.
+
+	This information can be used to determine what commit
+	was the tip of a branch "2 days ago".  This value is
+	false by default (no automated creation of log files).
 
 core.repositoryFormatVersion::
 	Internal variable identifying the repository format and layout

refs.c

@@ -721,7 +721,8 @@ static int log_ref_write(struct ref_lock *lock,
 	char *logrec;
 	const char *committer;
 
-	if (log_all_ref_updates) {
+	if (log_all_ref_updates &&
+	    !strncmp(lock->ref_name, "refs/heads/", 11)) {
 		if (safe_create_leading_directories(lock->log_file) < 0)
 			return error("unable to create directory for %s",
 				lock->log_file);