Merge branch 'dt/refs-backend-preamble'

In preparation for allowing different "backends" to store the refs
in a way different from the traditional "one ref per file in $GIT_DIR
or in a $GIT_DIR/packed-refs file" filesystem storage, reduce
direct filesystem access to ref-like things like CHERRY_PICK_HEAD
from scripts and programs.

* dt/refs-backend-preamble:
  git-stash: use update-ref --create-reflog instead of creating files
  update-ref and tag: add --create-reflog arg
  refs: add REF_FORCE_CREATE_REFLOG flag
  git-reflog: add exists command
  refs: new public ref function: safe_create_reflog
  refs: break out check for reflog autocreation
  refs.c: add err arguments to reflog functions

diff --cc Documentation/git-tag.txt
index 4b04c2b7d500521f1cf963a89ed010512faa29c6,231298065fa0d2a2c94b022f7e0ecc348479ce95..84f6496bf228454acaa04e570f7857ba1975cda4
--- 1/Documentation/git-tag.txt
--- 2/Documentation/git-tag.txt
+++ b/Documentation/git-tag.txt
@@@ -13,7 -13,8 +13,7 @@@ SYNOPSI
        <tagname> [<commit> | <object>]
  'git tag' -d <tagname>...
  'git tag' [-n[<num>]] -l [--contains <commit>] [--points-at <object>]
-       [--column[=<options>] | --no-column] [<pattern>...]
+       [--column[=<options>] | --no-column] [--create-reflog] [<pattern>...]
 -      [<pattern>...]
  'git tag' -v <tagname>...
  
  DESCRIPTION

diff --cc builtin/checkout.c
Simple merge

diff --cc builtin/update-ref.c
index 6763cf1837db99442a7140fe5bb87a0b3a1248fd,9046443731a3a4b14b4ab55b71686a2495155f87..04dd00f734166acc9c0670de061c4612877a7187
--- 1/builtin/update-ref.c
--- 2/builtin/update-ref.c
+++ b/builtin/update-ref.c
@@@ -422,14 -422,9 +429,15 @@@ int cmd_update_ref(int argc, const cha
        if (no_deref)
                flags = REF_NODEREF;
        if (delete)
 -              return delete_ref(refname, oldval ? oldsha1 : NULL, flags);
 +              /*
 +               * For purposes of backwards compatibility, we treat
 +               * NULL_SHA1 as "don't care" here:
 +               */
 +              return delete_ref(refname,
 +                                (oldval && !is_null_sha1(oldsha1)) ? oldsha1 : NULL,
 +                                flags);
        else
                return update_ref(msg, refname, sha1, oldval ? oldsha1 : NULL,
-                                 flags, UPDATE_REFS_DIE_ON_ERR);
+                                 flags | create_reflog_flag,
+                                 UPDATE_REFS_DIE_ON_ERR);
  }

diff --cc refs.c
Simple merge

diff --cc refs.h
Simple merge