Allow user to re-set the LHS file

This revision causes the "Select left side ..." context menu option
to always be available.  Selecting it will make the "Compare to ..."
option appear as well.  The "Compare to ..." option will disappear
once it is used.  This behavior more closely replicates the functionality
of the Windows explorer extension.
Merge pull request #1 from sparkchaser/re-set-lhs

Allow user to re-set the LHS file