hooks: always include HG_PENDING stable

hooks: always include HG_PENDING

Previously we would only include HG_PENDING in the hook args if the
transaction's writepending() actually wrote something. This is a bad criteria,
since it's possible that a previous call to writepending() wrote stuff and the
hooks want to still see that.

The solution is to always have hooks execute within the scope of the pending
changes by always putting HG_PENDING in the environment.

--- a/mercurial/hook.py	Tue Nov 03 12:31:33 2015 -0800
+++ b/mercurial/hook.py	Tue Nov 03 16:58:13 2015 -0800
@@ -122,7 +122,8 @@
     # make in-memory changes visible to external process
     tr = repo.currenttransaction()
     repo.dirstate.write(tr)
-    if tr and tr.writepending():
+    if tr:
+        tr.writepending()
         env['HG_PENDING'] = repo.root
 
     for k, v in args.iteritems():

--- a/tests/test-hook.t	Tue Nov 03 12:31:33 2015 -0800
+++ b/tests/test-hook.t	Tue Nov 03 16:58:13 2015 -0800
@@ -113,7 +113,7 @@
   $ hg pull ../a
   pulling from ../a
   searching for changes
-  prechangegroup hook: HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/a (glob)
+  prechangegroup hook: HG_PENDING=$TESTTMP/b HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/a (glob)
   adding changesets
   adding manifests
   adding file changes
@@ -272,7 +272,7 @@
   listkeys hook: HG_NAMESPACE=bookmarks HG_VALUES={'bar': '0000000000000000000000000000000000000000', 'foo': '0000000000000000000000000000000000000000'}
   no changes found
   pretxnopen hook: HG_TXNID=TXN:* HG_TXNNAME=push (glob)
-  prepushkey.forbid hook: HG_BUNDLE2=1 HG_KEY=baz HG_NAMESPACE=bookmarks HG_NEW=0000000000000000000000000000000000000000 HG_SOURCE=push HG_TXNID=TXN:* HG_URL=push (glob)
+  prepushkey.forbid hook: HG_BUNDLE2=1 HG_KEY=baz HG_NAMESPACE=bookmarks HG_NEW=0000000000000000000000000000000000000000 HG_PENDING=$TESTTMP/a HG_SOURCE=push HG_TXNID=TXN:* HG_URL=push (glob)
   pushkey-abort: prepushkey hook exited with status 1
   abort: exporting bookmark baz failed!
   [255]
@@ -306,7 +306,7 @@
   $ hg pull ../a
   pulling from ../a
   searching for changes
-  prechangegroup.forbid hook: HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/a (glob)
+  prechangegroup.forbid hook: HG_PENDING=$TESTTMP/b HG_SOURCE=pull HG_TXNID=TXN:* HG_URL=file:$TESTTMP/a (glob)
   abort: prechangegroup.forbid hook exited with status 1
   [255]