Merge branch 'ag/rebase-p'

Separate "rebase -p" codepath out of "rebase -i" implementation to
slim down the latter and make it easier to manage.

* ag/rebase-p:
  rebase: remove -p code from git-rebase--interactive.sh
  rebase: use the new git-rebase--preserve-merges.sh
  rebase: strip unused code in git-rebase--preserve-merges.sh
  rebase: introduce a dedicated backend for --preserve-merges

Author: gitster

.gitignore

@@ -119,6 +119,7 @@
 /git-rebase--helper
 /git-rebase--interactive
 /git-rebase--merge
+/git-rebase--preserve-merges
 /git-receive-pack
 /git-reflog
 /git-remote

Makefile

@@ -620,6 +620,7 @@ SCRIPT_LIB += git-mergetool--lib
 SCRIPT_LIB += git-parse-remote
 SCRIPT_LIB += git-rebase--am
 SCRIPT_LIB += git-rebase--interactive
+SCRIPT_LIB += git-rebase--preserve-merges
 SCRIPT_LIB += git-rebase--merge
 SCRIPT_LIB += git-sh-setup
 SCRIPT_LIB += git-sh-i18n
@@ -2397,6 +2398,7 @@ LOCALIZED_C = $(C_OBJ:o=c) $(LIB_H) $(GENERATED_H)
 LOCALIZED_SH = $(SCRIPT_SH)
 LOCALIZED_SH += git-parse-remote.sh
 LOCALIZED_SH += git-rebase--interactive.sh
+LOCALIZED_SH += git-rebase--preserve-merges.sh
 LOCALIZED_SH += git-sh-setup.sh
 LOCALIZED_PERL = $(SCRIPT_PERL)