Merge branch 'jk/describe-perf'

"git describe" optimization.

* jk/describe-perf:
  describe: split "found all tags" and max_candidates logic
  describe: stop traversing when we run out of names
  describe: stop digging for max_candidates+1
  t/perf: add tests for git-describe
  t6120: demonstrate weakness in disjoint-root handling

t/perf/p6100-describe.sh

@@ -0,0 +1,30 @@
+#!/bin/sh
+
+test_description='performance of git-describe'
+. ./perf-lib.sh
+
+test_perf_default_repo
+
+# clear out old tags and give us a known state
+test_expect_success 'set up tags' '
+	git for-each-ref --format="delete %(refname)" refs/tags >to-delete &&
+	git update-ref --stdin <to-delete &&
+	new=$(git rev-list -1000 HEAD | tail -n 1) &&
+	git tag -m new new $new &&
+	old=$(git rev-list       HEAD | tail -n 1) &&
+	git tag -m old old $old
+'
+
+test_perf 'describe HEAD' '
+	git describe HEAD
+'
+
+test_perf 'describe HEAD with one max candidate' '
+	git describe --candidates=1 HEAD
+'
+
+test_perf 'describe HEAD with one tag' '
+	git describe --match=new HEAD
+'
+
+test_done