Merge branch 'dl/rebase-i-keep-base'

"git rebase --keep-base <upstream>" tries to find the original base
of the topic being rebased and rebase on top of that same base,
which is useful when running the "git rebase -i" (and its limited
variant "git rebase -x").

The command also has learned to fast-forward in more cases where it
can instead of replaying to recreate identical commits.

* dl/rebase-i-keep-base:
rebase: teach rebase --keep-base
rebase tests: test linear branch topology
rebase: fast-forward --fork-point in more cases
rebase: fast-forward --onto in more cases
rebase: refactor can_fast_forward into goto tower
t3432: test for --no-ff's interaction with fast-forward
t3432: distinguish "noop-same" v.s. "work-same" in "same head" tests
t3432: test rebase fast-forward behavior
t3431: add rebase --fork-point tests

diff --cc Documentation/git-rebase.txt
index 7ecf766077ff68eb99ef780d628bdd20fff0558a,3146c1592deaa9790e143694bc9b5a07bb47d254..639a4179d18e4d154f30351aaa4f3b3c79c48c1e
--- 1/Documentation/git-rebase.txt
--- 2/Documentation/git-rebase.txt
+++ b/Documentation/git-rebase.txt
@@@ -543,6 -565,10 +565,8 @@@ In addition, the following pairs of opt
   * --preserve-merges and --interactive
   * --preserve-merges and --signoff
   * --preserve-merges and --rebase-merges
 - * --rebase-merges and --strategy
 - * --rebase-merges and --strategy-option
+  * --keep-base and --onto
+  * --keep-base and --root
  
  BEHAVIORAL DIFFERENCES
  -----------------------

diff --cc builtin/rebase.c
Simple merge

diff --cc contrib/completion/git-completion.bash
Simple merge

diff --cc t/t3400-rebase.sh
Simple merge

diff --cc t/t3404-rebase-interactive.sh
Simple merge