Fortunately for JENKINS-50693, MRP seems to set <tag> back to the expression it was before the release, so this needs no cleaning up.

Patched git-changelist-maven-extension in 1.0-alpha-2 to set an scmTag parameter, and using that as

<scm>
  <!-- … -->
  <tag>${scmTag}</tag>
</scm>

Confirmed that .flattened-pom.xml contains

• no tag when from a snapshot
• an actual tag when in a MRP artifact
• a commit hash when in Incrementals mode

Code changed in jenkins
User: Jesse Glick
Path:
pom.xml
http://jenkins-ci.org/commit/pom/3dd6e2d377daec76f58e50f47827760e8d390d13
Log:
JENKINS-50716 Flatten scm/tag.

Code changed in jenkins
User: Jesse Glick
Path:
pom.xml
http://jenkins-ci.org/commit/plugin-pom/3a7175bace6072f1075a9ded068e46985f282c7e
Log:
JENKINS-50716 Flatten scm/tag.

Code changed in jenkins
User: Jesse Glick
Path:
incrementals.md
http://jenkins-ci.org/commit/plugin-pom/c86bfaebc5179fdb42d4e7b71d87591f95466cbe
Log:
JENKINS-50716 Documentation for flattening of scm/tag.

Code changed in jenkins
User: Jesse Glick
Path:
.mvn/extensions.xml
pom.xml
http://jenkins-ci.org/commit/git-plugin/1654fd130b75209ed4b83fc3abc42e78530c5e93
Log:
JENKINS-50716 Flatten scm/tag.