GitcInit: fix gitc-init failure

Aligns argument usage of refactored GitcManifest (8c1e9cbef
"manifest_xml: refactor manifest parsing from client management") to fix
the `repo gitc-init` error: `fatal: manifest_file must be abspath`.

Change-Id: I1728032cce3f39ed1077bbb7ef714410c2c49e1a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/350374
Tested-by: Woody Lin <woodylin@google.com>
Reviewed-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Xin Li <delphij@google.com>

subcmds/gitc_init.py

@@ -68,7 +68,8 @@ use for this GITC client.
         sys.exit(1)
       manifest_file = opt.manifest_file
 
-    manifest = GitcManifest(self.repodir, gitc_client)
+    manifest = GitcManifest(self.repodir, os.path.join(self.client_dir,
+                                                       '.manifest'))
     manifest.Override(manifest_file)
     gitc_utils.generate_gitc_manifest(None, manifest)
     print('Please run `cd %s` to view your GITC client.' %