http-fetch: don't use double-slash as directory separator in URLs

http-fetch: don't use double-slash as directory separator in URLs

Please see http://bugs.debian.org/409887

http-fetch expected the URL given at the command line to have a trailing
slash anyway, and then added '/objects...' when requesting objects files
from the http server.

Now it doesn't require the trailing slash in <url> anymore, and strips
trailing slashes if given nonetheless.

Signed-off-by: Gerrit Pape <redacted>
Signed-off-by: Junio C Hamano <redacted>