prompt: squelch error output from cat

The files $g/rebase-{merge,apply}/{head-name,msgnum,end} are not
guaranteed to exist.  When attempting to cat them, squelch the error
output.

In addition to guarding against stray directories, this patch addresses
a real problem:

  # on terminal 1
  $ git rebase -i master
  # ignore editor, and switch to terminal 2
  cat: .git/rebase-merge/msgnum: No such file or directory
  cat: .git/rebase-merge/end: No such file or directory
  $

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>