JujutsuClient rewrites the short blob hashes in index lines from

jj diff --git into full hashes. The regex for the rest of the line

used \s.*, which also matched the trailing newline. jj leaves the file

mode off index lines for added and deleted files, so for those files the

newline was captured and written back out, followed by another newline.

The result was a blank line after the index line for every added or

deleted file.
Review Board's diff parser didn't care, so this wasn't too big a

problem.
The regex now only matches a space before the rest of the line. The

existing test for deleted files expected the blank line, so it has been

updated.

Ran unit tests.
Ran rbt diff 'root()' @- and verified that no index lines are

  followed by a blank line. Before this change, all 518 were.
Verified that modified files still keep the file mode on their index

  lines.