noCommit for PostReceive

This was noticed because it broke the datalad test suite, which pushed
to the remote and then fetched to check if it had received the expected
branches. Auto-init caused the git-annex branch on the remote to
diverge, breaking that test.

https://github.com/datalad/datalad/issues/1319#issuecomment-281649518

The auto-init still happens, it's staged in the journal, and will be
commited by some later git-annex command when it runs. Which is fine,
it's the same as that later command doing the auto-init.

This commit was supported by the NSF-funded DataLad project

Command/PostReceive.hs

@@ -13,8 +13,13 @@ import Git.Types
 import Annex.UpdateInstead
 import Command.Sync (mergeLocal, prepMerge, mergeConfig, getCurrBranch)
 
+-- This does not need to modify the git-annex branch to update the 
+-- work tree, but auto-initialization might change the git-annex branch.
+-- Since it would be surprising for a post-receive hook to make such a
+-- change, that's prevented by noCommit.
 cmd :: Command
-cmd = command "post-receive" SectionPlumbing
+cmd = noCommit $
+	command "post-receive" SectionPlumbing
 		"run by git post-receive hook"
 		paramNothing
 		(withParams seek)