vet: workaround buggy mac git grep behavior (#5716)

vet.sh

@@ -67,7 +67,9 @@ elif [[ "$#" -ne 0 ]]; then
 fi
 
 # - Ensure all source files contain a copyright message.
-not git grep -L "\(Copyright [0-9]\{4,\} gRPC authors\)\|DO NOT EDIT" -- '*.go'
+# (Done in two parts because Darwin "git grep" has broken support for compound
+# exclusion matches.)
+(grep -L "DO NOT EDIT" $(git grep -L "\(Copyright [0-9]\{4,\} gRPC authors\)" -- '*.go') || true) | fail_on_output
 
 # - Make sure all tests in grpc and grpc/test use leakcheck via Teardown.
 not grep 'func Test[^(]' *_test.go
@@ -81,7 +83,7 @@ not git grep -l 'x/net/context' -- "*.go"
 git grep -l '"math/rand"' -- "*.go" 2>&1 | not grep -v '^examples\|^stress\|grpcrand\|^benchmark\|wrr_test'
 
 # - Do not call grpclog directly. Use grpclog.Component instead.
-git grep -l 'grpclog.I\|grpclog.W\|grpclog.E\|grpclog.F\|grpclog.V' -- "*.go" | not grep -v '^grpclog/component.go\|^internal/grpctest/tlogger_test.go'
+git grep -l -e 'grpclog.I' --or -e 'grpclog.W' --or -e 'grpclog.E' --or -e 'grpclog.F' --or -e 'grpclog.V' -- "*.go" | not grep -v '^grpclog/component.go\|^internal/grpctest/tlogger_test.go'
 
 # - Ensure all ptypes proto packages are renamed when importing.
 not git grep "\(import \|^\s*\)\"github.com/golang/protobuf/ptypes/" -- "*.go"