git-svn: support for git-svn propset

git-svn: support for git-svn propset

This change allows git-svn to support setting subversion properties.

It is useful for manually setting properties when committing to a
subversion repo that *requires* properties to be set without requiring
moving your changeset to separate subversion checkout in order to
set props.

This change is initially from David Fraser, appearing at:

  http://mid.gmane.org/1927112650.1281253084529659.JavaMail.root@klofta.sjsoft.com>

They are now forward-ported to most recent git along with fixes to
deal with files in subdirectories.

Style and functional changes from Eric Wong have been taken
in their entirety from:

  http://mid.gmane.org/20141201094911.GA13931@dcvr.yhbt.net

There is a nit to point out: the code does not support
adding props unless there are also content changes to the files as
well.  This is demonstrated in the testcase.

[ew - simplify Git.pm usage for check-attr
    - improve shell portability for tests
    - minor phrasing changes in commit message]

Signed-off-by: David Fraser <davidf@sjsoft.com>
Signed-off-by: Alfred Perlstein <alfred@freebsd.org>
Signed-off-by: Eric Wong <normalperson@yhbt.net>