tests/test-simplemerge.py.out

changectx: increase perf of walk function

When running 'hg cat -r . <file>' it was doing an expensive ctx.walk(m) which
applied the regex to every file in the manifest.

This changes changectx.walk to iterate over just the files in the regex, if no
other patterns are specified. This cuts hg cat time by 50% in our repo and
probably benefits a few other commands as well.

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Ran 20 tests in 0.000s

OK