Merge branch 'rs/sha1-name-readdir-optim'

Optimize "what are the object names already taken in an alternate
object database?" query that is used to derive the length of prefix
an object name is uniquely abbreviated to.

* rs/sha1-name-readdir-optim:
  sha1_file: guard against invalid loose subdirectory numbers
  sha1_file: let for_each_file_in_obj_subdir() handle subdir names
  p4205: add perf test script for pretty log formats
  sha1_name: cache readdir(3) results in find_short_object_filename()

t/perf/p4205-log-pretty-formats.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+
+test_description='Tests the performance of various pretty format placeholders'
+
+. ./perf-lib.sh
+
+test_perf_default_repo
+
+for format in %H %h %T %t %P %p %h-%h-%h
+do
+	test_perf "log with $format" "
+		git log --format=\"$format\" >/dev/null
+	"
+done
+
+test_done