git-svn: add --ignore-paths option for fetching

git-svn: add --ignore-paths option for fetching

This will be useful when somebody want to checkout something partially from
repository with some non-standart layout or exclude some files from it.
Example: repository has structure /module-{a,b,c}/{trunk,branches,tags}/...
Modules are interdependent, and you want it to be single repostory (to commit
to all modules simultaneously and view complete history), but do not want
branches and tags be checked out into working copy.
Other use case is excluding some large blobs.

The quirk for now is that user must specify this option every fetch/rebase;
in other case he may get extra files or "file not found" errors. It may be
will be resolved by adding regular expression to .git/config into
[svn-remote ...] to make it persistent.

Signed-off-by: Vitaly "_Vi" Shukela <public_vi@tut.by>
Acked-by: Eric Wong <normalperson@yhbt.net>
[ew: replaced 4-space indent with tabs]
[ew: prefixed $ignore_regex with an underscore to be consistent
     with other globals in git-svn]
[ew: rearranged functions to minimize diff and removed prototype
     usage to be consistent with the rest of git-svn (and other
     Perl code in git (and they're ugly to me)]