git-svn: support rebase --preserve-merges

When git svn rebase is performed after an unpushed merge, the
rebase operation follows both parents and replays both the user's
local commits and those from the merged branch. This is usually
not the intended behavior.
This patch adds support for the --preserve-merges/-p flag which
allows for a better workflow by re-applying merge commits as merges.

[ew: fixed a minor syntax error]

Signed-off-by: Avishay Lavie <avishay.lavie@gmail.com>
Signed-off-by: Eric Wong <normalperson@yhbt.net>

Documentation/git-svn.txt

@@ -572,6 +572,8 @@ config key: svn.repackflags
 --merge::
 -s<strategy>::
 --strategy=<strategy>::
+-p::
+--preserve-merges::
 	These are only used with the 'dcommit' and 'rebase' commands.
 +
 Passed directly to 'git rebase' when using 'dcommit' if a

git-svn.perl

@@ -106,7 +106,7 @@ my ($_stdin, $_help, $_edit,
 	$_message, $_file, $_branch_dest,
 	$_template, $_shared,
 	$_version, $_fetch_all, $_no_rebase, $_fetch_parent,
-	$_merge, $_strategy, $_dry_run, $_local,
+	$_merge, $_strategy, $_preserve_merges, $_dry_run, $_local,
 	$_prefix, $_no_checkout, $_url, $_verbose,
 	$_git_format, $_commit_url, $_tag, $_merge_info, $_interactive);
 $Git::SVN::_follow_parent = 1;
@@ -255,6 +255,7 @@ my %cmd = (
 			  'local|l' => \$_local,
 			  'fetch-all|all' => \$_fetch_all,
 			  'dry-run|n' => \$_dry_run,
+			  'preserve-merges|p' => \$_preserve_merges,
 			  %fc_opts } ],
 	'commit-diff' => [ \&cmd_commit_diff,
 	                   'Commit a diff between two trees',
@@ -1592,6 +1593,7 @@ sub rebase_cmd {
 	push @cmd, '-v' if $_verbose;
 	push @cmd, qw/--merge/ if $_merge;
 	push @cmd, "--strategy=$_strategy" if $_strategy;
+	push @cmd, "--preserve-merges" if $_preserve_merges;
 	@cmd;
 }