This issue is scheduled to be addressed in the Vivado 2013.1 tool.
Until fixed, the suggested work-around is to use an external (third party) text editor when extensive use of the outlined operations is required.
Also, using smaller text files where possible is helpful. For example, constraints files can be broken up into timing and physical constraints files, and further broken up by regions of the device or hierarchy of the design in order to obtain smaller file sizes. Note that all required constraint files should be added to the active constraint set in order to be used.