fast-import: Don't use a maybe-clobbered errno value

Without this change, each diagnostic could use an errno value
clobbered by the close or unlink in rollback_lock_file.
Signed-off-by: NJim Meyering <meyering@redhat.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

fast-import.c

@@ -1541,9 +1541,10 @@ static void dump_marks(void)
 
 	f = fdopen(mark_fd, "w");
 	if (!f) {
+		int saved_errno = errno;
 		rollback_lock_file(&mark_lock);
 		failure |= error("Unable to write marks file %s: %s",
-			mark_file, strerror(errno));
+			mark_file, strerror(saved_errno));
 		return;
 	}
 
@@ -1556,16 +1557,18 @@ static void dump_marks(void)
 
 	dump_marks_helper(f, 0, marks);
 	if (ferror(f) || fclose(f)) {
+		int saved_errno = errno;
 		rollback_lock_file(&mark_lock);
 		failure |= error("Unable to write marks file %s: %s",
-			mark_file, strerror(errno));
+			mark_file, strerror(saved_errno));
 		return;
 	}
 
 	if (commit_lock_file(&mark_lock)) {
+		int saved_errno = errno;
 		rollback_lock_file(&mark_lock);
 		failure |= error("Unable to commit marks file %s: %s",
-			mark_file, strerror(errno));
+			mark_file, strerror(saved_errno));
 		return;
 	}
 }