I have experienced the same issue with Jenkins 1.596 / git plugin 2.3.4

I might have a working fix for this issue. Here is the fix: https://github.com/gennady/git-client-plugin/commit/aef7fff3ff765e2f8fd2b270d89e3f6b462cc2de

Give it a try if you don't mind.

You can compile the plugin yourself with

mvn package

or try already compiled version https://github.com/gennady/git-client-plugin/raw/8383bd7c222b52e26b0d1b395b2eb26766f86cf7/compiled-plugin/git-client.hpi

How to try:

• stop jenkins
• remove git-client, git-client.hpi, git-client.jpi from the plugins folder
• copy git-client.hpi to the plugins folder
• start jenkins

Code changed in jenkins
User: Gennady Trafimenkov
Path:
src/main/java/org/jenkinsci/plugins/gitclient/CliGitAPIImpl.java
src/test/java/org/jenkinsci/plugins/gitclient/GitAPITestCase.java
src/test/resources/unicodeCharsInChangelogRepo.zip
src/test/resources/unicodeCharsInChangelogRepoCreate.sh
http://jenkins-ci.org/commit/git-client-plugin/c99c91fcf497e784204398761be5c10f438d0e55
Log:
Fixed garbled commit messages on Windows

On windows changelog commit messages with unicode characters are
not saved correctly to changelog.xml when CliGitAPI
implementation is in use.

That happens because "git whatchanged" gives byte stream of data.
Commit messages in that stream are encoded in UTF-8. It is
necessary to explicitly decode bytestream to strings using UTF-8
encoding, otherwise default system encoding will be used.

This should fix issues:
https://issues.jenkins-ci.org/browse/JENKINS-6203
https://issues.jenkins-ci.org/browse/JENKINS-14798
https://issues.jenkins-ci.org/browse/JENKINS-23091

Fixed in git client plugin 1.19.3, released 6 Feb 2016