Fix progress text for "Interactively Rebase from Here"

"Preparing for rebase" can be used only before the interactive rebase dialog is shown,
afterwards it should be substituted with "Rebasing". However, it is a bit tedious to customize the progress text at that stage, so keeping it simple for now.

plugins/git4idea/src/git4idea/rebase/GitInteractiveRebaseAction.kt

@@ -33,7 +33,7 @@ class GitInteractiveRebaseAction : GitCommitEditingAction() {
     val project = e.project!!
     val repository = getRepository(e)
 
-    object : Task.Backgroundable(project, "Preparing to rebase") {
+    object : Task.Backgroundable(project, "Rebasing") {
       override fun run(indicator: ProgressIndicator) {
         val params = GitRebaseParams.editCommits(commit.parents.first().asString(), null, false)
         GitRebaseUtils.rebase(project, listOf(repository), params, indicator);