Merge branch 'jc/maint-co-track'

* jc/maint-co-track:
  Enhance hold_lock_file_for_{update,append}() API
  demonstrate breakage of detached checkout with symbolic link HEAD
  Fix "checkout --track -b newbranch" on detached HEAD

Conflicts:
	builtin-commit.c

diff --cc branch.c
Simple merge

diff --cc builtin-commit.c
index 33b659edce478fb0d3d4bb723e4f54c13db0efb1,b563a0d67cedc66825692863d9e57001fba35348..93ca49635731942c72cb52b27b1a6ba53becd1ca
--- 1/builtin-commit.c
--- 2/builtin-commit.c
+++ b/builtin-commit.c
@@@ -320,7 -320,8 +320,9 @@@ static char *prepare_index(int argc, co
                die("unable to write new_index file");
  
        fd = hold_lock_file_for_update(&false_lock,
-                                      git_path("next-index-%"PRIuMAX, (uintmax_t) getpid()), 1);
 -                                     git_path("next-index-%d", getpid()),
++                                     git_path("next-index-%"PRIuMAX,
++                                              (uintmax_t) getpid()),
+                                      LOCK_DIE_ON_ERROR);
  
        create_base_index();
        add_remove_files(&partial);

diff --cc builtin-fetch-pack.c
Simple merge

diff --cc builtin-revert.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc refs.c
Simple merge

diff --cc rerere.c
Simple merge

diff --cc sha1_file.c
Simple merge

diff --cc t/t7201-co.sh
index 82769b89fc79d91982420ddfc83ce6c3dd672095,f93478f97ff8d139d4c67690e1c85f098ca90a10..0e21632f19e75e1e7bbca1ceb2b9402a4d291584
--- 1/t/t7201-co.sh
--- 2/t/t7201-co.sh
+++ b/t/t7201-co.sh
@@@ -330,46 -330,28 +330,60 @@@ test_expect_success 
      test "$(git config branch.track2.merge)"
      git config branch.autosetupmerge false'
  
- test_expect_success \
-     'checkout w/--track from non-branch HEAD fails' '
-     git checkout -b delete-me master &&
-     rm .git/refs/heads/delete-me &&
-     test refs/heads/delete-me = "$(git symbolic-ref HEAD)" &&
-     test_must_fail git checkout --track -b track'
+ test_expect_success 'checkout w/--track from non-branch HEAD fails' '
+     git checkout master^0 &&
+     test_must_fail git symbolic-ref HEAD &&
+     test_must_fail git checkout --track -b track &&
+     test_must_fail git rev-parse --verify track &&
+     test_must_fail git symbolic-ref HEAD &&
+     test "z$(git rev-parse master^0)" = "z$(git rev-parse HEAD)"
+ '
+ 
+ test_expect_success 'detach a symbolic link HEAD' '
+     git checkout master &&
+     git config --bool core.prefersymlinkrefs yes &&
+     git checkout side &&
+     git checkout master &&
+     it=$(git symbolic-ref HEAD) &&
+     test "z$it" = zrefs/heads/master &&
+     here=$(git rev-parse --verify refs/heads/master) &&
+     git checkout side^ &&
+     test "z$(git rev-parse --verify refs/heads/master)" = "z$here"
+ '
  
 -test_expect_success 'checkout an unmerged path should fail' '
 +test_expect_success \
 +    'checkout with --track fakes a sensible -b <name>' '
 +    git update-ref refs/remotes/origin/koala/bear renamer &&
 +    git update-ref refs/new/koala/bear renamer &&
 +
 +    git checkout --track origin/koala/bear &&
 +    test "refs/heads/koala/bear" = "$(git symbolic-ref HEAD)" &&
 +    test "$(git rev-parse HEAD)" = "$(git rev-parse renamer)" &&
 +
 +    git checkout master && git branch -D koala/bear &&
 +
 +    git checkout --track refs/remotes/origin/koala/bear &&
 +    test "refs/heads/koala/bear" = "$(git symbolic-ref HEAD)" &&
 +    test "$(git rev-parse HEAD)" = "$(git rev-parse renamer)" &&
 +
 +    git checkout master && git branch -D koala/bear &&
 +
 +    git checkout --track remotes/origin/koala/bear &&
 +    test "refs/heads/koala/bear" = "$(git symbolic-ref HEAD)" &&
 +    test "$(git rev-parse HEAD)" = "$(git rev-parse renamer)" &&
 +
 +    git checkout master && git branch -D koala/bear &&
 +
 +    git checkout --track refs/new/koala/bear &&
 +    test "refs/heads/koala/bear" = "$(git symbolic-ref HEAD)" &&
 +    test "$(git rev-parse HEAD)" = "$(git rev-parse renamer)"
 +'
 +
 +test_expect_success \
 +    'checkout with --track, but without -b, fails with too short tracked name' '
 +    test_must_fail git checkout --track renamer'
 +
 +setup_conflicting_index () {
        rm -f .git/index &&
        O=$(echo original | git hash-object -w --stdin) &&
        A=$(echo ourside | git hash-object -w --stdin) &&