dirstate: don't write repo.currenttransaction to repo.dirstate if repo stable

dirstate: don't write repo.currenttransaction to repo.dirstate if repo
is None (issue4983)

Some hooks, such as post-init and post-clone, do not get a repo parameter in
their environment. If there is no repo, there is no repo.currenttransaction();
attempting to retrieve it anyway was causing crashes. Now currenttransaction is
only retrieved and written if the repo is not None.

--- a/mercurial/hook.py	Wed Dec 02 14:20:13 2015 -0800
+++ b/mercurial/hook.py	Thu Dec 03 01:38:21 2015 +0100
@@ -120,10 +120,11 @@
     env = {}
 
     # make in-memory changes visible to external process
-    tr = repo.currenttransaction()
-    repo.dirstate.write(tr)
-    if tr and tr.writepending():
-        env['HG_PENDING'] = repo.root
+    if repo is not None:
+        tr = repo.currenttransaction()
+        repo.dirstate.write(tr)
+        if tr and tr.writepending():
+            env['HG_PENDING'] = repo.root
 
     for k, v in args.iteritems():
         if callable(v):

--- a/tests/test-hook.t	Wed Dec 02 14:20:13 2015 -0800
+++ b/tests/test-hook.t	Thu Dec 03 01:38:21 2015 +0100
@@ -681,10 +681,19 @@
   $ hg tag -f foo
   ['a', 'foo', 'tip']
 
+post-init hooks must not crash (issue4983)
+This also creates the `to` repo for the next test block.
+
+  $ cd ..
+  $ cat << EOF >> hgrc-with-post-init-hook
+  > [hooks]
+  > post-init = printenv.py post-init
+  > EOF
+  $ HGRCPATH=hgrc-with-post-init-hook hg init to
+  post-init hook: HG_ARGS=init to HG_OPTS={'insecure': None, 'remotecmd': '', 'ssh': ''} HG_PATS=['to'] HG_RESULT=0
+
 new commits must be visible in pretxnchangegroup (issue3428)
 
-  $ cd ..
-  $ hg init to
   $ echo '[hooks]' >> to/.hg/hgrc
   $ echo 'prechangegroup = hg --traceback tip' >> to/.hg/hgrc
   $ echo 'pretxnchangegroup = hg --traceback tip' >> to/.hg/hgrc