pretty.c: add %f format specifier to format_commit_message()

pretty.c: add %f format specifier to format_commit_message()

This specifier represents the sanitized and filename friendly subject
line of a commit. No checks are made against the length of the string,
so users may need to trim the result to the desired length if using as a
filename. This is commonly used by format-patch to massage commit
subjects into filenames and output patches to files.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>