FileContents.getLines performance fix

[isopov]

Inspired by #136 and #346 part. I was testing on the spring-boot project with the following config:

<module name="TreeWalker">
    <module name="LeftCurly"/>
    <module name="FallThrough"/>
    <module name="CustomImportOrder"/>
    <module name="LineLength"/>
    <module name="GenericWhitespace"/>
    <module name="NoWhitespaceAfter"/>
    <module name="NoWhitespaceBefore"/>
    <module name="OperatorWrap"/>
    <module name="WhitespaceAfter"/>
    <module name="WhitespaceAround"/>
</module>

Using checkstyle before this fix gived me 71 MB of String[] allocations. After this fix Only 2.4 MB of String[] were allocated. Execution time (that is not stable and was not properly measured) reduced insignificantly - both executions were less than 5s.

[daniilyar]

I've tested this, fix looks good and really reduces garbage overhead.

[romani]

merged as FF, thanks a lot.