rebase: introduce a dedicated backend for --preserve-merges

This duplicates git-rebase--interactive.sh to
git-rebase--preserve-merges.sh. This is done to split -p from -i. No
modifications are made to this file here, but any code that is not used
by -p will be stripped in the next commit.

Signed-off-by: Alban Gruin <alban.gruin@gmail.com>

.gitignore

@@ -117,6 +117,7 @@
 /git-rebase--helper
 /git-rebase--interactive
 /git-rebase--merge
+/git-rebase--preserve-merges
 /git-receive-pack
 /git-reflog
 /git-remote

Makefile

@@ -582,6 +582,7 @@ 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
@@ -2271,6 +2272,7 @@ 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)