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/commit.c b/builtin/commit.c
index 1d805f5..eda0d32 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1695,10 +1695,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		if (!reflog_msg)
 			reflog_msg = "commit (merge)";
 		pptr = commit_list_append(current_head, pptr);
-		fp = fopen(git_path_merge_head(), "r");
-		if (fp == NULL)
-			die_errno(_("could not open '%s' for reading"),
-				  git_path_merge_head());
+		fp = xfopen(git_path_merge_head(), "r");
 		while (strbuf_getline_lf(&m, fp) != EOF) {
 			struct commit *parent;