Fix for mercurial-4.2

[hattya]

hgext.color is merged into core in Mercurial 4.2.

[hattya]

This does not work with <mercurial-4.2. I will investigate it.

[coveralls]

Coverage remained the same at 95.722% when pulling 8a65f68 on hattya:hg42 into 7bd797f on tk0miya:master.

[coveralls]

Coverage remained the same at 95.722% when pulling 9122451 on hattya:hg42 into 7bd797f on tk0miya:master.

[sbraz]

@tk0miya Hi, is there any chance to see this merged? Do you still maintain this project?

[maxgrenderjones]

Note that if you get bored waiting, you can always run pip install git+https://github.com/hattya/diff-highlight@hg42#egg=diff_highlight