pull: update docs & code for option compatibility with rebasing

git-pull.txt includes merge-options.txt, which is written assuming
merges will happen.  git-pull has allowed rebases for many years; update
the documentation to reflect that.

While at it, pass any `--signoff` flag through to the rebase backend too
so that we don't have to document it as merge-specific.  Rebase has
supported the --signoff flag for years now as well.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge.txt

@@ -61,6 +61,8 @@ merge has resulted in conflicts.
 
 OPTIONS
 -------
+:git-merge: 1
+
 include::merge-options.txt[]
 
 -m <msg>::

Documentation/git-pull.txt

@@ -19,13 +19,12 @@ Incorporates changes from a remote repository into the current branch.
 If the current branch is behind the remote, then by default it will
 fast-forward the current branch to match the remote.  If the current
 branch and the remote have diverged, the user needs to specify how to
-reconcile the divergent branches with `--no-ff`, `--ff`, or `--rebase`
-(or the corresponding configuration options in `pull.ff` or
-`pull.rebase`).
+reconcile the divergent branches with `--rebase` or `--no-rebase` (or
+the corresponding configuration option in `pull.rebase`).
 
 More precisely, `git pull` runs `git fetch` with the given parameters
 and then depending on configuration options or command line flags,
-will call either `git merge` or `git rebase` to reconcile diverging
+will call either `git rebase` or `git merge` to reconcile diverging
 branches.
 
 <repository> should be the name of a remote repository as
@@ -136,7 +135,7 @@ published that history already.  Do *not* use this option
 unless you have read linkgit:git-rebase[1] carefully.
 
 --no-rebase::
-	Override earlier --rebase.
+	This is shorthand for --rebase=false.
 
 Options related to fetching
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~

Documentation/merge-options.txt

@@ -2,6 +2,9 @@
 --no-commit::
 	Perform the merge and commit the result. This option can
 	be used to override --no-commit.
+ifdef::git-pull[]
+	Only useful when merging.
+endif::git-pull[]
 +
 With --no-commit perform the merge and stop just before creating
 a merge commit, to give the user a chance to inspect and further
@@ -39,6 +42,7 @@ set to `no` at the beginning of them.
 	to `MERGE_MSG` before being passed on to the commit machinery in the
 	case of a merge conflict.
 
+ifdef::git-merge[]
 --ff::
 --no-ff::
 --ff-only::
@@ -47,6 +51,22 @@ set to `no` at the beginning of them.
 	default unless merging an annotated (and possibly signed) tag
 	that is not stored in its natural place in the `refs/tags/`
 	hierarchy, in which case `--no-ff` is assumed.
+endif::git-merge[]
+ifdef::git-pull[]
+--ff-only::
+	Only update to the new history if there is no divergent local
+	history.  This is the default when no method for reconciling
+	divergent histories is provided (via the --rebase=* flags).
+
+--ff::
+--no-ff::
+	When merging rather than rebasing, specifies how a merge is
+	handled when the merged-in history is already a descendant of
+	the current history.  If merging is requested, `--ff` is the
+	default unless merging an annotated (and possibly signed) tag
+	that is not stored in its natural place in the `refs/tags/`
+	hierarchy, in which case `--no-ff` is assumed.
+endif::git-pull[]
 +
 With `--ff`, when possible resolve the merge as a fast-forward (only
 update the branch pointer to match the merged branch; do not create a
@@ -55,9 +75,11 @@ descendant of the current history), create a merge commit.
 +
 With `--no-ff`, create a merge commit in all cases, even when the merge
 could instead be resolved as a fast-forward.
+ifdef::git-merge[]
 +
 With `--ff-only`, resolve the merge as a fast-forward when possible.
 When not possible, refuse to merge and exit with a non-zero status.
+endif::git-merge[]
 
 -S[<keyid>]::
 --gpg-sign[=<keyid>]::
@@ -73,6 +95,9 @@ When not possible, refuse to merge and exit with a non-zero status.
 	In addition to branch names, populate the log message with
 	one-line descriptions from at most <n> actual commits that are being
 	merged. See also linkgit:git-fmt-merge-msg[1].
+ifdef::git-pull[]
+	Only useful when merging.
+endif::git-pull[]
 +
 With --no-log do not list one-line descriptions from the
 actual commits being merged.
@@ -102,10 +127,17 @@ With --no-squash perform the merge and commit the result. This
 option can be used to override --squash.
 +
 With --squash, --commit is not allowed, and will fail.
+ifdef::git-pull[]
++
+Only useful when merging.
+endif::git-pull[]
 
 --no-verify::
 	This option bypasses the pre-merge and commit-msg hooks.
 	See also linkgit:githooks[5].
+ifdef::git-pull[]
+	Only useful when merging.
+endif::git-pull[]
 
 -s <strategy>::
 --strategy=<strategy>::
@@ -127,6 +159,10 @@ With --squash, --commit is not allowed, and will fail.
 	default trust model, this means the signing key has been signed by
 	a trusted key.  If the tip commit of the side branch is not signed
 	with a valid key, the merge is aborted.
+ifdef::git-pull[]
++
+Only useful when merging.
+endif::git-pull[]
 
 --summary::
 --no-summary::
@@ -166,3 +202,7 @@ endif::git-pull[]
 	projects that started their lives independently. As that is
 	a very rare occasion, no configuration variable to enable
 	this by default exists and will not be added.
+ifdef::git-pull[]
++
+Only useful when merging.
+endif::git-pull[]

builtin/pull.c

@@ -893,6 +893,8 @@ static int run_rebase(const struct object_id *newbase,
 	strvec_pushv(&args, opt_strategy_opts.v);
 	if (opt_gpg_sign)
 		strvec_push(&args, opt_gpg_sign);
+	if (opt_signoff)
+		strvec_push(&args, opt_signoff);
 	if (opt_autostash == 0)
 		strvec_push(&args, "--no-autostash");
 	else if (opt_autostash == 1)