As Jeff King pointed out, some placeholder expansions are related to
each other: the steps to calculate one go most of the way towards
calculating the other, too.

This patch makes format_commit_message() parse the commit message
only once, remembering the position of each item.  This speeds up
handling of format strings containing multiple placeholders from the
set %s, %a*, %c*, %e, %b.

Here are the timings for the git version in next.  The first one is
to estimate the overhead of the caching, the second one is taken
from http://svn.tue.mpg.de/tentakel/trunk/tentakel/Makefile as an
example of a format string found in the wild.  The times are the
fastest of three consecutive runs in each case:

$ time git log --pretty=format:%e >/dev/null

real    0m0.381s
user    0m0.340s
sys     0m0.024s

$ time git log --pretty=format:"* %cd %cn%n%n%s%n%b" >/dev/null

real    0m0.623s
user    0m0.556s
sys     0m0.052s

And here the times with this patch:

$ time git log --pretty=format:%e >/dev/null

real    0m0.385s
user    0m0.332s
sys     0m0.040s

$ time git log --pretty=format:"* %cd %cn%n%n%s%n%b" >/dev/null

real    0m0.563s
user    0m0.504s
sys     0m0.048s
Signed-off-by: NRene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>