Merge branch 'hf/unpack-trees-quadratic-scan' into next

The cache-scanning loop in 'next_cache_entry()' has been optimized
to avoid rescanning already-unpacked index entries, preventing a
quadratic performance slow-down when diffing the working tree
against a commit with a pathspec matching early index entries.

* hf/unpack-trees-quadratic-scan:
  unpack-trees: avoid quadratic index scan in next_cache_entry()

t/perf/p0009-diff-pathspec.sh

@@ -0,0 +1,32 @@
+#!/bin/sh
+
+test_description='Tests performance of diffing the working tree with a pathspec'
+
+. ./perf-lib.sh
+
+test_perf_fresh_repo
+
+count=10000
+if test_have_prereq EXPENSIVE
+then
+	count=100000
+fi
+
+# The entries exist only in the index, which is enough to
+# exercise the index scan.
+test_expect_success 'setup' '
+	blob=$(echo content | git hash-object -w --stdin) &&
+	{
+		printf "100644 $blob\taaa/file\n" &&
+		printf "100644 $blob\tf%s\n" $(test_seq $count)
+	} | git update-index --index-info &&
+	git commit -q -m initial &&
+	mkdir -p aaa &&
+	echo content >aaa/file
+'
+
+test_perf 'diff pathspec subtree' '
+	git diff HEAD -- aaa/file
+'
+
+test_done

unpack-trees.c

@@ -671,8 +671,10 @@ static struct cache_entry *next_cache_entry(struct unpack_trees_options *o)
 
 	while (pos < index->cache_nr) {
 		struct cache_entry *ce = index->cache[pos];
-		if (!(ce->ce_flags & CE_UNPACKED))
+		if (!(ce->ce_flags & CE_UNPACKED)) {
+			o->internal.cache_bottom = pos;
 			return ce;
+		}
 		pos++;
 	}
 	return NULL;