log: teach "terminator" vs "separator" mode to "--pretty=format"

log: teach "terminator" vs "separator" mode to "--pretty=format"

This attached patch introduces a single bit "use_terminator" in "struct
rev_info", which is normally false (i.e. most formats use separator
semantics) but by flipping it to true, you can ask for terminator
semantics just like oneline format does.

The function get_commit_format(), which is what parses "--pretty=" option,
now takes a pointer to "struct rev_info" and updates its commit_format and
use_terminator fields.  It used to return the value of type "enum
cmit_fmt", but all the callers assigned it to rev->commit_format.

There are only two cases the code turns use_terminator on.  Obviously, the
traditional oneline format (--pretty=oneline) is one of them, and the new
case is --pretty=tformat:... that acts like --pretty=format:... but flips
the bit on.

With this, "--pretty=tformat:%H %s" acts like --pretty=oneline.

Signed-off-by: Junio C Hamano <gitster@pobox.com>