Quote reference names while fetching with curl.

Quote reference names while fetching with curl.

curl_escape ought to do this, but we should not let it quote
slashes (nobody said refs/tags cannot have subdirectories), so
we roll our own safer version.  With this, the last part of
git-clone from Martin's moodle repository that used to fail now
works, which reads:

 $ git-http-fetch -v -a -w 'tags/MOODLE_15_MERGED **INVALID**' \
   'tags/MOODLE_15_MERGED **INVALID**' \
   http://locke.catalyst.net.nz/git/moodle.git/

Signed-off-by: Junio C Hamano <junkio@cox.net>