patchgit-svn: propset support v3

Added:

Eric Sunshine's:
- suggestion to include the comment about propset only
  working on files with content changes.

Eric Wong's:
- use && to chain commands throughout tests
- use svn_cmd wrapper throughout tests
- show $! in die messages
- favor $(...) over `...` in tests
- make new_props an array simplify building the final list
- wrap long comments (help output still needs fixing)
- remove unnecessary FIXME comment
- commit message improvements.

Torsten Bögershausen:
- Fixes for test cases.

Finally, I have refactored the test cases to reduce duplicate code such that further fixes and improvements can be done in one place.

JFYI, branches are here:
1st review changes:
  https://github.com/splbio/git/compare/master-git-svn-propset-upstream?expand=1
Squashed into:
  https://github.com/splbio/git/compare/master-git-svn-propset-upstream-one-diff?expand=1

Current review in different commits:
  https://github.com/splbio/git/compare/master-git-svn-propset-upstream2?expand=1
Squashed into:
  https://github.com/splbio/git/compare/master-git-svn-propset-upstream-one-diff2?expand=1
  
Thank you very much.

Alfred Perlstein (1):
  git-svn: Support for git-svn propset

 git-svn.perl               | 49 ++++++++++++++++++++++-
 perl/Git/SVN/Editor.pm     | 42 ++++++++++++++++++++
 t/t9148-git-svn-propset.sh | 97 ++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 187 insertions(+), 1 deletion(-)
 create mode 100755 t/t9148-git-svn-propset.sh