doc: describe git svn init --ignore-refs

Add the missing documentation for `git svn init --ignore-refs`.
Signed-off-by: NAndreas Heiduk <asheiduk@gmail.com>
Signed-off-by: NEric Wong <e@80x24.org>

Documentation/git-svn.txt

@@ -95,6 +95,10 @@ If you still want the old default, you can get it by passing
 `--prefix ""` on the command line (`--prefix=""` may not work if
 your Perl's Getopt::Long is < v2.37).
 
+--ignore-refs=<regex>;;
+	When passed to 'init' or 'clone' this regular expression will
+	be preserved as a config key.  See 'fetch' for a description
+	of `--ignore-refs`.
 --ignore-paths=<regex>;;
 	When passed to 'init' or 'clone' this regular expression will
 	be preserved as a config key.  See 'fetch' for a description
@@ -138,6 +142,18 @@ the same local time zone.
 --parent;;
 	Fetch only from the SVN parent of the current HEAD.
 
+--ignore-refs=<regex>;;
+	Ignore refs for branches or tags matching the Perl regular
+	expression. A "negative look-ahead assertion" like
+	`^refs/remotes/origin/(?!tags/wanted-tag|wanted-branch).*$`
+	can be used to allow only certain refs.
++
+[verse]
+config key: svn-remote.<name>.ignore-refs
++
+If the ignore-refs configuration key is set, and the command-line
+option is also given, both regular expressions will be used.
+
 --ignore-paths=<regex>;;
 	This allows one to specify a Perl regular expression that will
 	cause skipping of all matching paths from checkout from SVN.