test: add a push race case where the racing client create a new head

We check case where the raced client push updates an existing head while the
racing client push creates a new one.

tests/test-push-race.t

@@ -278,3 +278,109 b' Check the result of the push'
   |/
   @  842e2fac6304 C-ROOT (default)
   
+Pushing while someone creates a new head
+-----------------------------------------
+
+Pushing a new changeset while someone creates a new branch.
+
+#  a (raced)
+#  |
+#  * b
+#  |/
+#  *
+
+(resync-all)
+
+  $ hg -R ./server pull ./client-racy
+  pulling from ./client-racy
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  (run 'hg update' to get a working copy)
+  $ hg -R ./client-other pull
+  pulling from ssh://user@dummy/server
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  (run 'hg update' to get a working copy)
+  $ hg -R ./client-racy pull
+  pulling from ssh://user@dummy/server
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  (run 'hg update' to get a working copy)
+
+  $ hg -R server graph
+  o  59e76faf78bd C-D (default)
+  |
+  o  a9149a1428e2 C-B (default)
+  |
+  | o  51c544a58128 C-C (default)
+  | |
+  | o  98217d5a1659 C-A (default)
+  |/
+  @  842e2fac6304 C-ROOT (default)
+  
+
+Creating changesets
+
+(new head)
+
+  $ hg -R client-other/ up 'desc("C-A")'
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ echo aaa >> client-other/a
+  $ hg -R client-other/ commit -m "C-E"
+  created new head
+
+(children of existing head)
+
+  $ hg -R client-racy/ up 'desc("C-C")'
+  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ echo bbb >> client-racy/a
+  $ hg -R client-racy/ commit -m "C-F"
+
+Pushing
+
+  $ hg -R client-racy push -r 'tip' > ./push-log 2>&1 &
+
+  $ waiton $TESTTMP/readyfile
+
+  $ hg -R client-other push -fr 'tip'
+  pushing to ssh://user@dummy/server
+  searching for changes
+  remote: adding changesets
+  remote: adding manifests
+  remote: adding file changes
+  remote: added 1 changesets with 1 changes to 1 files (+1 heads)
+
+  $ release $TESTTMP/watchfile
+
+Check the result of the push
+
+  $ cat ./push-log
+  pushing to ssh://user@dummy/server
+  searching for changes
+  wrote ready: $TESTTMP/readyfile
+  waiting on: $TESTTMP/watchfile
+  abort: push failed:
+  'repository changed while pushing - please try again'
+
+  $ hg -R server graph
+  o  d603e2c0cdd7 C-E (default)
+  |
+  | o  51c544a58128 C-C (default)
+  |/
+  o  98217d5a1659 C-A (default)
+  |
+  | o  59e76faf78bd C-D (default)
+  | |
+  | o  a9149a1428e2 C-B (default)
+  |/
+  @  842e2fac6304 C-ROOT (default)
+