format-patch: add option to generate CC list

For git-format-patch(1), the --cc option appends a specified email address to be CC'd for all patches outputted. Additionally, via format.cc, CC recipients can be configured. A user may want to select recipients in a more programatic manner though.

Introduce the --cc-cmd flag and format.ccCmd configuration to optionally specify a program to execute and provide additional email addresses to CC for each patch. The program is expected to output a single email address per line of stdout.