rebase: fail-fast the pull if working dir is not clean (BC)

Refuse to run 'hg pull --rebase' if there are uncommitted changes:
so that instead of going ahead with fetching changes and then suddenly aborting
the rebase, we can warn user of uncommitted changes (or unclean repo state)
right up front.
In tests, we create a 'histedit' session to verify that also an unfinished
state is detected and handled.

hgext/rebase.py

@@ -1316,6 +1316,10 b' def pullrebase(orig, ui, repo, *args, **'
                 ui.debug('--update and --rebase are not compatible, ignoring '
                          'the update flag\n')
 
+            ui.debug('before rebase: ensure working dir is clean\n')
+            cmdutil.checkunfinished(repo)
+            cmdutil.bailifchanged(repo)
+
             revsprepull = len(repo)
             origpostincoming = commands.postincoming
             def _dummy(*args, **kwargs):

tests/test-rebase-pull.t

@@ -1,6 +1,7 b''
   $ cat >> $HGRCPATH <<EOF
   > [extensions]
   > rebase=
+  > histedit=
   > 
   > [alias]
   > tglog = log -G --template "{rev}: '{desc}' {branches}\n"
@@ -72,6 +73,27 b' Re-run:'
   searching for changes
   no changes found
 
+Abort pull early if working dir is not clean:
+
+  $ echo L1-mod > L1
+  $ hg pull --rebase
+  abort: uncommitted changes
+  [255]
+  $ hg update --clean --quiet
+
+Abort pull early if another operation (histedit) is in progress:
+
+  $ hg histedit . -q --commands - << EOF
+  > edit d80cc2da061e histedit: generate unfinished state
+  > EOF
+  Editing (d80cc2da061e), you may commit or record as needed now.
+  (hg histedit --continue to resume)
+  [1]
+  $ hg pull --rebase
+  abort: histedit in progress
+  (use 'hg histedit --continue' or 'hg histedit --abort')
+  [255]
+  $ hg histedit --abort --quiet
 
 Invoke pull --rebase and nothing to rebase: