.gitignore: explicitly allow paths

This prevents git from picking up masterdirs that don't happen to be
named the usual way and more custom and temporary files that should not
be tracked by git.

.gitignore

@@ -2,13 +2,18 @@
 *.swp
 *~
 \#*#
-*.o
+
-masterdir*
+# exclude everything in root except files and directories from void-packages
-hostdir*
+/*
-masterdir*/
+!/CONTRIBUTING.md
-hostdir*/
+!/COPYING
+!/Manual.md
+!/README.md
+!/common
+!/etc
+!/srcpkgs
+!/xbps-src
 etc/conf
 etc/conf.*
 etc/virtual
 etc/xbps.d/custom
-.xbps-checkvers*.plist