Add "--early-output" log flag for interactive GUI use

Add "--early-output" log flag for interactive GUI use

This adds support for "--early-output[=n]" as a flag to the "git log"
family of commands.  This allows GUI programs to state that they want to
get some output early, in order to be able to show at least something
quickly, even if the full output may take longer to generate.

If no count is specified, a default count of a hundred commits will be
used, although the actual numbr of commits output may be smaller
depending on how many commits were actually found in the first tenth of
a second (or if *everything* was found before that, in which case no
early output will be provided, and only the final list is made
available).

When the full list is generated, there will be a "Final output:" string
prepended to it, regardless of whether any early commits were shown or
not, so that the consumer can always know the difference between early
output and the final list.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>