use xfopen() in more places

xfopen()

 - provides error details
 - explains error on reading, or writing, or whatever operation
 - has l10n support
 - prints file name in the error

Some of these are missing in the places that are replaced with xfopen(),
which is a clear win. In some other places, it's just less code (not as
clearly a win as the previous case but still is).

The only slight regresssion is in remote-testsvn, where we don't report
the file class (marks files) in the error messages anymore. But since
this is a _test_ svn remote transport, I'm not too concerned.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/am.c b/builtin/am.c
index a95dd8b..f5dac77 100644
--- a/builtin/am.c
+++ b/builtin/am.c
@@ -1275,12 +1275,8 @@ static int parse_mail(struct am_state *state, const char *mail)
 		die("BUG: invalid value for state->scissors");
 	}
 
-	mi.input = fopen(mail, "r");
-	if (!mi.input)
-		die("could not open input");
-	mi.output = fopen(am_path(state, "info"), "w");
-	if (!mi.output)
-		die("could not open output 'info'");
+	mi.input = xfopen(mail, "r");
+	mi.output = xfopen(am_path(state, "info"), "w");
 	if (mailinfo(&mi, am_path(state, "msg"), am_path(state, "patch")))
 		die("could not parse patch");