abuild: add 'index' subcommand to refresh the abuild repo index

this is a convenience synonym to update_abuildrepo_index

abuild.in

@@ -1811,6 +1811,7 @@ undeps (){
 # compat
 installdeps() { deps; }
 uninstalldeps() { undeps; }
+index() { update_abuildrepo_index; }
 
 all() {
 	if ! [ -n "$force" ]; then
@@ -1917,6 +1918,7 @@ Commands:
   cleanpkg    Remove already built binary and source package
   deps        Install packages listed in makedepends and depends
   fetch       Fetch sources to \$SRCDEST and verify checksums
+  index       Regenerate the APKINDEX for abuildrepo
   listpkg     List target packages
   package     Create package in \$PKGDEST
   prepare     Apply patches