"Auth fail" error with maven-scm-provider-jgit during release:prepare - maven

Question

I am on a windows machine where git command line is not installed. I have installed GitExtensions and an EGit and this is sufficient for me (until now).
Now I wanted to perform a maven release to BinTray, following this guide:
Publishing releases using Github, Bintray and maven-release-plugin
Running:
mvn -Prelease release:prepare
Produces this output:
[INFO] ------------------------------------------------------------------------
[INFO] Building xxxxx 1.0.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] --- maven-release-plugin:2.4.1:prepare (default-cli) # xxxxx ---
[INFO] Resuming release from phase 'scm-commit-release'
[INFO] Checking in modified POMs...
[INFO] Executing: cmd.exe /X /C "git add -- pom.xml"
[INFO] Working directory: C:\Users\jbr\git\yyyyyyyyyyyyyyy
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 1.770 s
[INFO] Finished at: 2015-09-12T20:17:24+02:00
[INFO] Final Memory: 20M/989M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-release-plugin:2.4.1:prepare (default-cli) on project xxxxx: Unable to commit files
[ERROR] Provider message:
[ERROR] The git-add command failed.
[ERROR] Command output:
[ERROR] 'git' is not recognized as an internal or external command,
[ERROR] operable program or batch file.
Absolutely fair, I never use the git command from the windows shell and I never set up the pageant.exe stuff. (for details see this answer to the question “Why git can't remember my passphrase under Windows”).
My second idea was to use the maven-scm-provider-jgit to not depend on the git command line.
The output is now:
[INFO] ------------------------------------------------------------------------
[INFO] Building xxxxx 1.0.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] --- maven-release-plugin:2.4.1:prepare (default-cli) # xxxxx ---
[INFO] Change the default 'git' provider implementation to 'jgit'.
[INFO] Resuming release from phase 'scm-commit-release'
[INFO] Checking in modified POMs...
[INFO] push changes to remote... refs/heads/master:refs/heads/master
[INFO] fetch url: git#github.com:jmini/yyyyyyyyyyyyyyy.git
[INFO] push url: git#github.com:jmini/yyyyyyyyyyyyyyy.git
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-release-plugin:2.4.1:prepare (default-cli) on project xxxxx: An error is occurred in the checkin process: Exception while executing SCM command. JGit checkin failure! git#github.com:jmini/yyyyyyyyyyyyyyy.git: Auth fail -> [Help 1]
Much better, but still no luck.
My guess is that the private key and the passphrase are not loaded. How should I configure them?
This answer suggests that for some SCM providers, it is possible to specify the credentials in the <servers> section of settings.xml. The domain name of the repository should be used as <id>.
According to the last comment with scm:git:ssh://git#bitbucket.org/{account}/{project} used in the <developerConnection>, this entry is working in the settings.xml:
<server>
<id>bitbucket.org</id>
<privateKey>/c/Users/neil.hunt/.ssh/id_rsa</privateKey>
</server>
Is this construct supported by the JGit SCM provider?
When searching for “Auth failed” error, I found this related question (plain JGit, no maven plugin):
How do you set the configuration for jschconfigsessionfactory for jgit so that pull and push work?