Add automatic binary file detection to diff and export
Based on a patch by Fuming Wang
- add util.binary which decides whether a file is binary if it has any NUL
characters in the first 1K.
- teach mdiff.unidiff to print "binary file <x> has changed" for binary files
- add text flag to cause unidiff and dodiff to treat all files as text
- add -a and --text flags (like diff(1)) to hg diff and export
- update docs