Merge branch 'jc/format-patch-error-check'

"git format-patch" used overwrite an existing patch/cover-letter
file.  A new "--no-clobber" option stops it.

* jc/format-patch-error-check:
format-patch: notice failure to open cover letter for writing
builtin/log: downcase the beginning of error messages

diff --cc builtin/log.c
Simple merge