trace: correct variable name in write() error message

Our error message for write() always mentions GIT_TRACE,
even though we may be writing for a different variable
entirely. It's also not quite accurate to say "fd given by
GIT_TRACE environment variable", as we may hit this error
based on a filename the user put in the variable (we do
complain and switch to stderr if the file cannot be opened,
but it's still possible to hit a write() error on the
descriptor later).

So let's fix those things, and switch to our more usual
"unable to do X: Y" format for the error.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

trace.c

@@ -93,9 +93,6 @@ void trace_disable(struct trace_key *key)
 	key->need_close = 0;
 }
 
-static const char err_msg[] = "could not trace into fd given by "
-	"GIT_TRACE environment variable";
-
 static int prepare_trace_line(const char *file, int line,
 			      struct trace_key *key, struct strbuf *buf)
 {
@@ -133,8 +130,11 @@ static int prepare_trace_line(const char *file, int line,
 
 static void trace_write(struct trace_key *key, const void *buf, unsigned len)
 {
-	if (write_in_full(get_trace_fd(key), buf, len) < 0)
-		warning("%s: write error (%s)", err_msg, strerror(errno));
+	if (write_in_full(get_trace_fd(key), buf, len) < 0) {
+		normalize_trace_key(&key);
+		warning("unable to write trace for %s: %s",
+			key->key, strerror(errno));
+	}
 }
 
 void trace_verbatim(struct trace_key *key, const void *buf, unsigned len)