style: Fixup strange semantics in hg m5style

style: Fixup strange semantics in hg m5style

The 'hg m5style' command had some rather strange semantics. When
called without arguments, it applied the style checker to all added
files and modified regions of modified files. However, when providing
a list of files, it used that list as an ignore list instead of
specifically checking those files.

This patch makes the m5style command behave more like other Mercurial
commands where the arguments are used to specify which files to work
on instead of which files to ignore.