sequencer: introduce a helper to read files written by scripts

As we are slowly teaching the sequencer to perform the hard work for
the interactive rebase, we need to read files that were written by
shell scripts.

These files typically contain a single line and are invariably ended
by a line feed (and possibly a carriage return before that). Let's use
a helper to read such files and to remove the line ending.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -234,6 +234,40 @@ static int write_message(struct strbuf *msgbuf, const char *filename)
 	return 0;
 }
 
+/*
+ * Reads a file that was presumably written by a shell script, i.e. with an
+ * end-of-line marker that needs to be stripped.
+ *
+ * Note that only the last end-of-line marker is stripped, consistent with the
+ * behavior of "$(cat path)" in a shell script.
+ *
+ * Returns 1 if the file was read, 0 if it could not be read or does not exist.
+ */
+static int read_oneliner(struct strbuf *buf,
+	const char *path, int skip_if_empty)
+{
+	int orig_len = buf->len;
+
+	if (!file_exists(path))
+		return 0;
+
+	if (strbuf_read_file(buf, path, 0) < 0) {
+		warning_errno(_("could not read '%s'"), path);
+		return 0;
+	}
+
+	if (buf->len > orig_len && buf->buf[buf->len - 1] == '\n') {
+		if (--buf->len > orig_len && buf->buf[buf->len - 1] == '\r')
+			--buf->len;
+		buf->buf[buf->len] = '\0';
+	}
+
+	if (skip_if_empty && buf->len == orig_len)
+		return 0;
+
+	return 1;
+}
+
 static struct tree *empty_tree(void)
 {
 	return lookup_tree(EMPTY_TREE_SHA1_BIN);