strbuf_expand: convert "%%" to "%"

strbuf_expand: convert "%%" to "%"

The only way to safely quote arbitrary text in a pretty-print user
format is to replace instances of "%" with "%x25". This is slightly
unreadable, and many users would expect "%%" to produce a single
"%", as that is what printf format specifiers do.

This patch converts "%%" to "%" for all users of strbuf_expand():

 (1) git-daemon interpolated paths

 (2) pretty-print user formats

 (3) merge driver command lines

Case (1) was already doing the conversion itself outside of
strbuf_expand(). Case (2) is the intended beneficiary of this patch.
Case (3) users probably won't notice, but as this is user-facing
behavior, consistently providing the quoting mechanism makes sense.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>