Order: mergePaths should run before collapseGroups

[fregante]

An SVG like this one:

<g>
    <path fill="#AC162C" d="pathdata"/>
    <path fill="#AC162C" d="pathdata-2"/>
</g>

…will be compressed into:

<g fill="#AC162C">
    <path d="pathdata1+2"/>
</g>

… which, if svgoed again, will be optimized further:

<path d="pathdata1+2" fill="#AC162C"/>

This is due to collapseGroups running before mergePaths, therefore not catching the newly-unnecessary <g fill="#AC162C">

Test SVG file: http://cl.ly/Yw2Y

[coveralls]

Coverage remained the same when pulling 81d7a08 on bfred-it:master into 259ae7e on svg:master.

[GreLI]

There is a catch. Consider the example:

<g>
    <path fill="#AC162C" d="pathdata"/>
    <g>
        <path fill="#AC162C" d="pathdata-2"/>
    </g>
</g>

Right now it'll be transformed into

<g>
    <path fill="#AC162C" d="pathdata"/>
    <path fill="#AC162C" d="pathdata-2"/>
</g>

after colapseGroups and then merged with mergePath.
But it will not happen after your changes. A more generic solution is needed.
For now there is --multipass key to account such cases as yours.