Use --rebase instead of --no-edit in gitPull

git pull --no-edit fails with exit 128 on divergent branches because
fast-forward is impossible.  Using --rebase handles divergent histories
by rebasing local commits on top of fetched remote commits, producing
a linear history without merge commits.

- src/Converge.hs: Change gitPull from --no-edit to --rebase
- test/GitPullSpec.hs: Update rebase test to expect success and
  verify linear history (3 commits, no merge parents)
- test/*.hs: Clean up unused imports

src/Converge.hs

@@ -305,7 +305,7 @@ gitPull :: RepoConfig -> IO (ExitCode, Text, Text)
 gitPull repo =
     runGitIn
         (rcPath repo)
-        ["pull", "--no-edit", T.unpack (rcRemote repo), T.unpack (rcBranch repo)]
+        ["pull", "--rebase", T.unpack (rcRemote repo), T.unpack (rcBranch repo)]
 
 -- | Check if the repository currently has merge conflicts.
 hasConflicts :: FilePath -> IO Bool