want_restart_for_use_change: backtrack only if needed (bug 632598)

Only backtrack if changes break a USE dependency. Prior
to this fix, the included test case succeeded only with
a --backtrack setting of 3 or more, but now it succeeds
with a --backtrack setting of 2.

X-Gentoo-bug: 632598
X-Gentoo-bug-url: https://bugs.gentoo.org/632598
Acked-by: Brian Dolbec <dolsen@gentoo.org>
2 files changed