git-svn: implement git svn create-ignore

git-svn: implement git svn create-ignore

git svn create-ignore (to create one .gitignore per directory
from the svn:ignore properties.  This has the disadvantage of
committing the .gitignore during the next dcommit, but when you
import a repo with tons of ignores (>1000), using git svn show-ignore
to build .git/info/exclude is *not* a good idea, because things like
git-status will end up doing >1000 fnmatch *per file* in the repo,
which leads to git-status taking more than 4s on my Core2Duo 2Ghz 2G
RAM)

	* git-svn.perl (%cmd): Add the new command `create-ignore'.
	(&cmd_create_ignore): New.
	* t/t9101-git-svn-props.sh: Adjust the test-case for show-ignore and
	add a test case for create-ignore.

[ew: added commit message from
  <05CAB148-56ED-4FF1-8AAB-4BA2A0B70C2C@lrde.epita.fr> ]

Signed-off-by: Benoit Sigoure <tsuna@lrde.epita.fr>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>