patchgit-svn: add hook to allow modifying the subversion commit message

Documentation/githooks.txt, git-svn.perl, templates/hooks--git-svn-msg.sample

diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index 28edefa..dd90c1a 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -45,6 +45,25 @@ the commit after inspecting the message file.
 The default 'applypatch-msg' hook, when enabled, runs the
 'commit-msg' hook, if the latter is enabled.
 
+git-svn-msg
+~~~~~~~~~~~
+
+This hook is invoked by 'git svn' when creating a git commit
+from a subversion commit.  It takes a single parameter, the name
+of the file that holds the original subversion commit message.
+Exiting with non-zero status causes 'git svn' to abbort before
+creating the git commit. Additional, the environment variable
+`GIT_SVN_METADATA=:` is set to the string that would appear
+after the 'git-svn-id:' line.
+
+The hook is allowed to edit the message file in place, and can
+be used to normalize the message into some project standard
+format (if the project has one). It can also be used to refuse
+the commit after inspecting the message file.
+
+The default 'git-svn-msg' hook, when enabled, runs the
+'commit-msg' hook, if the latter is enabled.
+
 pre-applypatch
 ~~~~~~~~~~~~~~
 
diff --git a/git-svn.perl b/git-svn.perl
index b67fef0..030516c 100755
--- a/git-svn.perl
+++ b/git-svn.perl
@@ -3007,6 +3007,31 @@ sub do_git_commit {
 		require Encode;
 		Encode::from_to($log_entry->{log}, 'UTF-8', $enc);
 	}
+
+	# execute commit message hook if present
+	if (-x "$ENV{GIT_DIR}/hooks/git-svn-msg") {
+		print "Calling commit mesasage hook\n";
+
+		# write commit message to file
+		my $msg_file = "$ENV{GIT_DIR}/SVN_COMMIT_EDITMSG";
+		mkfile($msg_file);
+		open my $fh, '>', $msg_file or croak $!;
+		binmode $fh;
+		print $fh $log_entry->{log} or croak $!;
+		close $fh or croak $!;
+
+		$ENV{GIT_SVN_METADATA} = $log_entry->{metadata};
+		system("$ENV{GIT_DIR}/hooks/git-svn-msg", $msg_file);
+		die "git-svn-msg hook failed: $!\n" if $?;
+
+		# read commit message back
+		open $fh, '<', $msg_file or croak $!;
+		binmode $fh;
+		$log_entry->{log} = do { local $/; <$fh> };
+		close $fh or croak $!;
+		unlink $msg_file;
+	}
+
 	print $msg_fh $log_entry->{log} or croak $!;
 	restore_commit_header_env($old_env);
 	unless ($self->no_metadata) {
diff --git a/templates/hooks--git-svn-msg.sample b/templates/hooks--git-svn-msg.sample
new file mode 100755
index 0000000..6584fd8
--- /dev/null
+++ b/templates/hooks--git-svn-msg.sample
@@ -0,0 +1,18 @@
+#!/bin/sh
+#
+# An example hook script to check the commit log message taken by
+# git svn from a subversion commit.
+#
+# The hook should exit with non-zero status after issuing an
+# appropriate message if it wants to stop the commit.  The hook is
+# allowed to edit the commit message file.
+#
+# GIT_SVN_METADATA will contain the same info as printed after
+# the git-svn-id line.
+#
+# To enable this hook, rename this file to "git-svn-msg".
+
+. git-sh-setup
+test -x "$GIT_DIR/hooks/commit-msg" &&
+	exec "$GIT_DIR/hooks/commit-msg" ${1+"$@"}
+: