git-prompt.sh: remove annoying error messages

Sometimes, one can go to a project directory to see logs or diffs, or
whatever he wants, during interactive rebase (when rebase editor is open
in other terminal window). And he can see annoying error messages in a
shell:

    cat: .git/rebase-merge/msgnum: No such file or directory
    cat: .git/rebase-merge/end: No such file or directory

This patch supresses this messages.

Signed-off-by: Alexey Shumkin <Alex.Crezoff@gmail.com>