Print errno upon failure to open the COMMIT_EDITMSG file

When the COMMIT_EDITMSG cannot be opened, give more information to the user
by giving the 'errno' information.

Signed-off-by: Cristian Peraferrer <corellian.c@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-commit.c

@@ -502,7 +502,8 @@ static int prepare_to_commit(const char *index_file, const char *prefix)
 
 	fp = fopen(git_path(commit_editmsg), "w");
 	if (fp == NULL)
-		die("could not open %s", git_path(commit_editmsg));
+		die("could not open %s: %s",
+		    git_path(commit_editmsg), strerror(errno));
 
 	if (cleanup_mode != CLEANUP_NONE)
 		stripspace(&sb, 0);