Modifying an existing commit message is not supported.
You can provide an initial commit message with a ICommitMessageProvider registered through the org.eclipse.egit.ui.commitMessageProvider extension point.
For instance:
package org.example.myapp;
import org.eclipse.core.resources.IResource;
import org.eclipse.egit.ui.ICommitMessageProvider;
public class DummyCommitMessageProvider implements ICommitMessageProvider {
@Override
public String getMessage(IResource[] resources) {
return "This is a dummy message\n";
}
}
and in your bundle's plugin.xml:
<extension
id="org.example.myapp.commitMessage"
point="org.eclipse.egit.ui.commitMessageProvider">
<commitMessageProvider
class="org.example.myapp.DummyCommitMessageProvider">
</commitMessageProvider>
</extension>