Build: Properly nest gitignore for luci-go

The directory tools/luci-go is now under a subtreed repo mirror and
should have its own gitignore file.

Bug: chromium:754168
Change-Id: I716b6c853f04c9284287e417500a821c01dab8c4
Reviewed-on: https://chromium-review.googlesource.com/707146
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507430}

.gitignore

@@ -235,9 +235,6 @@ vs-chromium-project.txt
 /sync/sync.xml
 /sync_testserver.log
 /testserver.log
-/tools/luci-go/linux64/isolate
-/tools/luci-go/mac64/isolate
-/tools/luci-go/win64/isolate.exe
 # See third_party/.gitignore for entries covering src/third_party.
 /tools/.bisect-builds-cache.json
 /tools/distcc

tools/luci-go/.gitignore

@@ -0,0 +1,3 @@
+/linux64/isolate
+/mac64/isolate
+/win64/isolate.exe