contrib: new tool to convert svn:externals to Git submodules

This is meant to be a post-processing step after SVN-to-Git conversion
by SubGit (https://subgit.com/), which creates a ".gitsvnextmodules"
file that we will use for svn:externals conversion.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Elijah Newren <newren@gmail.com>

Author: marcows

contrib/filter-repo-demos/README.md

@@ -16,6 +16,7 @@ lint-history         |Run some lint command on all non-binary files in history.
 clean-ignore         |Delete files from history which match current gitignore rules.
 filter-lamely (or filter‑branch‑ish) |A nearly bug compatible re-implementation of filter-branch (the git testsuite passes using it instead of filter-branch), with some performance tricks to make it several times faster (though it's still glacially slow compared to filter-repo).
 bfg-ish              |A re-implementation of most of BFG Repo Cleaner, with new features and bug fixes.
+convert-svnexternals |Insert Git submodules according to SVN externals.
 
 ## Purpose