svn: assume URLs from the command-line are URI-encoded

svn: assume URLs from the command-line are URI-encoded

And then unescape them when writing to $GIT_CONFIG.

SVN has different rules for repository URLs (usually the root)
and for paths within that repository (below the HTTP layer).
Thus, for the request URI path at the HTTP level, the URI needs
to be encoded.  However, in the body of the HTTP request (the
with underlying SVN XML protocol), those paths should not be
URI-encoded[1].  For non-HTTP(S) requests, SVN appears to be
more flexible and will except weird characters in the URL as
well as URI-encoded ones.

Since users are used to using URLs being entirely URI-encoded,
git svn will now attempt to unescape the path portion of URLs
while leaving the actual repository URL untouched.

This change will be reflected in newly-created $GIT_CONFIG files
only.  This allows users to switch between svn(+ssh)://, file://
and http(s):// urls without changing the fetch/branches/tags
config keys.  This won't affect existing imports at all (since
things didn't work before this commit anyways), and will allow
users to force escaping into repository paths that look like
they're escaped (but are not).

Thanks to Mike Smullin for the original bug report and Björn
Steinbrink for summarizing it into testable cases for me.

[1] Except when committing copies/renames, see
    commit 29633bb91c7bcff31ff3bb59378709e3e3ef627d

Signed-off-by: Eric Wong <normalperson@yhbt.net>