BSD grep doesn't support -P

Test: New grep command tested on Linux and macOS
Bug: b/206150971
Change-Id: I9f1684298672fee1561166a03392bd07be8c5cd6

tools/aosp/aosp_sha.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 LOCAL_DIR="$( dirname "${BASH_SOURCE}" )"
 
-if git branch -vv | grep -q -P "^\*[^\[]+\[aosp/"; then
+if git branch -vv | grep -q -E "^\*[^\[]+\[aosp/"; then
     # Change appears to be in AOSP
     exit 0
 elif git log -n 1 --format='%B' $1 | grep -q -E "^Ignore-AOSP-First: .+" ; then