obsolescence: add test utility for the "branch replacement" logic during push

Mercurial checks for the introduction of new heads on push. Evolution comes
into play to detect if existing branches on the server are being replaced by
some of the new one we push.

The current code for this logic is very basic (eg: issue4354) and was poorly
tested. We have a better implementation coming in the evolve extension fixing
these issues and with more serious tests coverage. In the process of upstreaming
this improved logic, we start with adding the test case that are already passing
with the current implementation. Once they are all in, we'll upstream the better
implementation and the extra test case.

This changeset introduce the common setup script used by these tests.

tests/testlib/push-checkheads-util.sh

@@ -0,0 +1,44 b''
+# setup config and various utility to test new heads checks on push
+
+cat >> $HGRCPATH <<EOF
+[ui]
+# simpler log output
+logtemplate ="{node|short} ({phase}): {desc}\n"
+
+[phases]
+# non publishing server
+publish=False
+
+[extensions]
+# we need to strip some changeset for some test cases
+strip=
+
+[experimental]
+# enable evolution
+evolution=all
+
+[alias]
+# fix date used to create obsolete markers.
+debugobsolete=debugobsolete -d '0 0'
+EOF
+
+mkcommit() {
+   echo "$1" > "$1"
+   hg add "$1"
+   hg ci -m "$1"
+}
+
+getid() {
+   hg log --hidden --template '{node}\n' --rev "$1"
+}
+
+setuprepos() {
+    echo creating basic server and client repo
+    hg init server
+    cd server
+    mkcommit root
+    hg phase --public .
+    mkcommit A0
+    cd ..
+    hg clone server client
+}