init: restore default --manifest-name

The merge of the repo & init parser missed this default.

When running `repo init ...` in an existing checkout but w/out the -m
option, then repo would error out complaining that -m is required when
it didn't do this before.

Change-Id: I58035d48cc413b5d373702b9dc3b9ecd3fd1e900
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/303945
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Jonathan Nieder <jrn@google.com>
diff --git a/repo b/repo
index cf9f186..d9c97de 100755
--- a/repo
+++ b/repo
@@ -298,7 +298,7 @@
                    help='manifest repository location', metavar='URL')
   group.add_option('-b', '--manifest-branch', metavar='REVISION',
                    help='manifest branch or revision (use HEAD for default)')
-  group.add_option('-m', '--manifest-name',
+  group.add_option('-m', '--manifest-name', default='default.xml',
                    help='initial manifest file', metavar='NAME.xml')
   group.add_option('-g', '--groups', default='default',
                    help='restrict manifest projects to ones with specified '