chore: fix incomplete sanitization

[s0]

Summary

This PR fixes an alert that was found on LGTM: https://lgtm.com/projects/g/yarnpkg/yarn/alerts/?mode=tree&ruleFocus=1506222917439

yarn/src/util/filter.js

Lines 103 to 104 in 8cbbc6d

	// the micromatch regex expects unix path separators
	loc = loc.replace('\\', '/');

This replacement was incomplete, and would only replace the first backslash which is not what was intended.

Test plan

Output from node:

> 'fooo\\bar\\baz'.replace('\\', '/')
'fooo/bar\\baz'
> 'fooo\\bar\\baz'.replace(/\\/g, '/')
'fooo/bar/baz'

I'm not sure how to otherwise test this part of the code, or introduce a test case that proves that the previous version fails and the fix fixes it.

[batebobo]

Looks good!