am: indicate where a failed patch is to be found

am: indicate where a failed patch is to be found

If "git am" fails to apply something, the end user may need to know
where to find the patch that failed to apply, so that the user can
do other things (e.g. trying "GNU patch" on it, running "diffstat"
to see what it tried to change, etc.)  The input to "am" may have
contained more than one patch, or the message may have been MIME
encoded, and knowing what the user fed to "am" does not help very
much for this purpose.

Also introduce advice.amworkdir configuration to allow people who
learned where to look to squelch this message.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>