Fix glob cache conflict when excludes=nil and excludes=[]string{}

Performing the same glob twice, once with excludes=nil and once
with excludes=[]string{} would hit the same entry in the glob
cache (since the glob filename would be the same), but fail
the verifyGlob check because DeepEqual considers []string(nil)
and []string{} to be different.  Use a manual array check
instead.

Test: glob_test.go
Change-Id: If0d4fe80163a871077b7276e1b4a3e888a4a4898