update-bash: fully resolve Git's path in hash.

Otherwise Bash can cache a relative PATH and then get upset when it
changes directory and cannot find it any more.

Closes Homebrew/homebrew#48493.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/cmd/update-bash.sh

@@ -271,6 +271,7 @@ EOS
       odie "Git must be installed and in your PATH!"
     fi
   fi
+  hash -p "$(cd "$(dirname "$(which_git)")" && pwd -P)/git" git
 
   if [[ -z "$HOMEBREW_VERBOSE" ]]
   then