sparse-revlog: add a `trim_endidx` function in C

We are about to implement a native version of `slicechunktodensity`. For
clarity, we introduce the helper functions first.

This function implement a subpart of the python function `_trimchunk` in
`mercurial/revlogutils/deltas.py`. Handling of actual Python objects is left
to the caller function.

mercurial/cext/revlog.c

@@ -1078,6 +1078,24 b' index_segment_span(indexObject *self, Py'
 	return (end_offset - start_offset) + (int64_t)end_size;
 }
 
+/* returns revs[startidx:endidx] without empty trailing revs */
+static Py_ssize_t trim_endidx(indexObject *self, const Py_ssize_t *revs,
+                              Py_ssize_t startidx, Py_ssize_t endidx)
+{
+	int length;
+	while (endidx > 1 && endidx > startidx) {
+		length = index_get_length(self, revs[endidx - 1]);
+		if (length < 0) {
+			return -1;
+		}
+		if (length != 0) {
+			break;
+		}
+		endidx -= 1;
+	}
+	return endidx;
+}
+
 static inline int nt_level(const char *node, Py_ssize_t level)
 {
 	int v = node[level >> 1];