context: safegaurd against 'lx' being passed as file flag in manifest Subversion can have a file as executable link. When using hgsubversion, we will have both islink and isexec True. This will lead to _flags being set to `lx`. However, manifest expects flag to be one-byte so it will crash if 'lx' is passed. Also it's impossible to have an executable link. This patch will safegaurd us from having 'lx' being a possible value. This was authored by Ivan Lezhankin from Yandex. Differential Revision: https://phab.mercurial-scm.org/D3985