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

test/TestHelper.hs

@@ -22,7 +22,6 @@ import qualified Data.Text as T
 import System.Directory (createDirectory)
 import System.Exit (ExitCode)
 import System.FilePath ((</>))
-import System.IO (writeFile)
 import System.IO.Temp (withSystemTempDirectory)
 import System.Process (readProcessWithExitCode)