Git: Block [*_]HEAD for branch name

Can have false positives, but I think it's better than tracking all of
git's internal names.

See https://stackoverflow.com/a/17596689/764870

Change-Id: Ifa3d942c4f791e5f8542e0cc174b934f2a95cbcd
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
Reviewed-by: André Hartmann <aha_1980@gmx.de>

src/plugins/git/gitplugin.cpp

@@ -193,6 +193,7 @@ QString GitPlugin::invalidBranchAndRemoteNamePattern()
         "|^[/-]"  // no leading slash or dash
         "|\""     // no quotes
         "|\\*"    // no asterisk
+        "|(^|[A-Z]+_)HEAD" // no HEAD, FETCH_HEAD etc.
     );
 }