Merge branch 'jc/push-to-checkout'

Extending the js/push-to-deploy topic, the behaviour of "git push"
when updating the working tree and the index with an update to the
branch that is checked out can be tweaked by push-to-checkout hook.

* jc/push-to-checkout:
  receive-pack: support push-to-checkout hook
  receive-pack: refactor updateInstead codepath

Documentation/config.txt

@@ -2158,11 +2158,15 @@ receive.denyCurrentBranch::
 	message. Defaults to "refuse".
 +
 Another option is "updateInstead" which will update the working
-directory (must be clean) if pushing into the current branch. This option is
+tree if pushing into the current branch.  This option is
 intended for synchronizing working directories when one side is not easily
 accessible via interactive ssh (e.g. a live web site, hence the requirement
 that the working directory be clean). This mode also comes in handy when
 developing inside a VM to test and fix code on different Operating Systems.
++
+By default, "updateInstead" will refuse the push if the working tree or
+the index have any difference from the HEAD, but the `push-to-checkout`
+hook can be used to customize this.  See linkgit:githooks[5].
 
 receive.denyNonFastForwards::
 	If set to true, git-receive-pack will deny a ref update which is

Documentation/githooks.txt

@@ -341,6 +341,36 @@ Both standard output and standard error output are forwarded to
 'git send-pack' on the other end, so you can simply `echo` messages
 for the user.
 
+push-to-checkout
+~~~~~~~~~~~~~~~~
+
+This hook is invoked by 'git-receive-pack' on the remote repository,
+which happens when a 'git push' is done on a local repository, when
+the push tries to update the branch that is currently checked out
+and the `receive.denyCurrentBranch` configuration variable is set to
+`updateInstead`.  Such a push by default is refused if the working
+tree and the index of the remote repository has any difference from
+the currently checked out commit; when both the working tree and the
+index match the current commit, they are updated to match the newly
+pushed tip of the branch.  This hook is to be used to override the
+default behaviour.
+
+The hook receives the commit with which the tip of the current
+branch is going to be updated.  It can exit with a non-zero status
+to refuse the push (when it does so, it must not modify the index or
+the working tree).  Or it can make any necessary changes to the
+working tree and to the index to bring them to the desired state
+when the tip of the current branch is updated to the new commit, and
+exit with a zero status.
+
+For example, the hook can simply run `git read-tree -u -m HEAD "$1"`
+in order to emulate 'git fetch' that is run in the reverse direction
+with `git push`, as the two-tree form of `read-tree -u -m` is
+essentially the same as `git checkout` that switches branches while
+keeping the local changes in the working tree that do not interfere
+with the difference between the branches.
+
+
 pre-auto-gc
 ~~~~~~~~~~~