Clarify that test rebases are not kept

There has been a long-standing misconception that git-review pushes
automatically rebased changes by default. It does not, but our
documentation and context help have been less than clear on that
point, contributing to this impression. Try to do a better job of
explaining that the default rebasing performed by git-review is
purely exploratory, and used only to notify users about possible
merge conflicts with their target branch before pushing a change.

Change-Id: I3c841af5ff9430a0de4d9dc9526dd3be6ab53ad2

doc/source/usage.rst

@@ -30,7 +30,8 @@ If you want to submit a branch for review and then remove the local branch::
 
     git review -f
 
-If you want to skip the automatic "git rebase -i" step::
+If you want to be able to push a change which has a merge conflict with the
+remote branch::
 
     git review -R
 

git-review.1

@@ -131,12 +131,17 @@ If the master branch is different enough, the rebase can produce merge conflicts
 If that happens rebasing will be aborted and diff displayed for not\-rebased branches.
 You can also use
 .Ar \-\-no\-rebase ( Ar \-R )
-to always skip rebasing.
+to always skip the test rebase.
 .It Fl f , Fl \-finish
 Close down the local branch and switch back to the target branch on
 successful submission.
 .It Fl F , Fl \-force\-rebase
-Force a rebase before doing anything else, even if not otherwise needed.
+Force a rebase before doing anything else, even if not otherwise needed. Unlike
+the normal test rebase, this rebase will be kept. Use if you know you want to
+push a commit automatically rebased onto the current state of the target
+branch. This is discouraged if amending an existing change, since it creates
+unnecessary additional differences from the previous revision and so makes
+things harder for reviewers of your changes.
 .It Fl n , Fl \-dry\-run
 Don\(aqt actually perform any commands that have direct effects. Print them
 instead.
@@ -171,10 +176,14 @@ one patch.
 .It Fl v , Fl \-verbose
 Turns on more verbose output.
 .It Fl R , Fl \-no\-rebase
-Do not automatically perform a rebase before submitting the change to
-Gerrit.
+Don't test for possible merge conflicts with the target branch before pushing.
+If the test rebase detects no merge conflicts then the rebase is undone and
+your previous state is pushed. If merge conflicts are detected git\-review
+exits with the rebase in progress allowing you to address it manually. By
+default git\-review will never push the results of a rebase without your
+explicit involvement.
 .Pp
-When submitting a change for review, you will usually want it to be based on the tip of upstream branch in order to avoid possible conflicts. When amending a change and rebasing the new patchset, the Gerrit web interface will show a difference between the two patchsets which contains all commits in between. This may confuse many reviewers that would expect to see a much simpler difference.
+Use the this option to skip the merge conflict test, allowing you to push merge conflicts.
 .Pp
 Also can be used for
 .Fl \-compare

git_review/cmd.py

@@ -1540,10 +1540,12 @@ additional information:
     rebase_group = parser.add_mutually_exclusive_group()
     rebase_group.add_argument("-R", "--no-rebase", dest="rebase",
                               action="store_false",
-                              help="Don't rebase changes before submitting.")
+                              help="Don't test for remote merge conflicts"
+                                   " before pushing.")
     rebase_group.add_argument("-F", "--force-rebase", dest="force_rebase",
                               action="store_true",
-                              help="Force rebase even when not needed.")
+                              help="Force and push a rebase even when not"
+                                   " needed.")
 
     track_group = parser.add_mutually_exclusive_group()
     track_group.add_argument("--track", dest="track",