git-svn: strip off leading slashes on --trunk argument

git-svn: strip off leading slashes on --trunk argument

The following command

 git svn clone \
	-r9500:10006 \
	svn://svn.debian.org/svn/pkg-games/packages \
	--trunk=/trunk/freedoom \
	--branches=/branches/freedoom \
	--tags=/tags/freedoom \
	freedoom.git.2009091

produces strange results:

With v1.6.3.3 (and perhaps earlier versions), this would fetch up to
and including r9978 (the last revision of the no_iwad_alternatives
branch before it was deleted), check it out, and prematurely declare
success, leaving out some commits to the trunk (r9984, r9985, r10006)
from after the branch was merged.

With v1.6.5-rc0~74 (svn: allow branches outside of refs/remotes,
2009-08-11) and later, this fetches up to and including r9978 and then
attempts a post-fetch checkout and fails.

 r9978 = 25f0920175c395f0f22f54ae7a2318147f745274
 (refs/remotes/no_iwad_alternatives)
 fatal: refs/remotes/trunk: not a valid SHA1
 update-ref refs/heads/master refs/remotes/trunk: command returned error: 128

Checking .git/config reveals

 fetch = packages//trunk/freedoom:refs/remotes/trunk

And with both 1.6.3.3 and 1.7.1, using --trunk=trunk/freedom without
the leading slash (/) works fine.

Moral: git-svn needs to scrub an initial / from $_trunk and related
arguments it receives.  Make it so.

Reported-by: Jon Dowland <jmtd@debian.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Eric Wong <normalperson@yhbt.net>