pushrace: avoid crash on bare push when using concurrent push mode

If the remote is empty, we do now bother computing head changes and the
'pushbranchmap' attribute stays at None.

We now handle and tests this case.

mercurial/exchange.py

@@ -730,7 +730,8 b' def _pushb2ctxcheckheads(pushop, bundler'
     # * if we don't push anything, there are nothing to check.
     if not pushop.force and pushop.outgoing.missingheads:
         allowunrelated = 'related' in bundler.capabilities.get('checkheads', ())
-        if not allowunrelated:
+        emptyremote = pushop.pushbranchmap is None
+        if not allowunrelated or emptyremote:
             bundler.newpart('check:heads', data=iter(pushop.remoteheads))
         else:
             affected = set()

tests/test-push.t

@@ -297,3 +297,22 b' Test push hook locking'
   lock:  user *, process * (*s) (glob)
   wlock: user *, process * (*s) (glob)
 
+Test bare push with multiple race checking options
+--------------------------------------------------
+
+  $ hg init test-bare-push-no-concurrency
+  $ hg init test-bare-push-unrelated-concurrency
+  $ hg -R test-revflag push -r 0 test-bare-push-no-concurrency --config server.concurrent-push-mode=strict
+  pushing to test-bare-push-no-concurrency
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  $ hg -R test-revflag push -r 0 test-bare-push-unrelated-concurrency --config server.concurrent-push-mode=check-related
+  pushing to test-bare-push-unrelated-concurrency
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files