When we are using git-svn or git-p4, we post-process the diff to change it from

something that looks like a git diff to something that looks like a subversion

diff or perforce diff.
This works most of the time, but our handling of the index lines had some

unintended consequences. If the file in question had a change to a line

starting with two hyphens and a space ('-- '), then the unified diff will

represent that as '--- old text / '+-- new text'. Since we only looked for

'--- filename' when scanning for file headers, we'd modify these lines that

were actually content, and the patch would fail to apply. I've changed this to

make sure that we only munge a line starting with '--- ' if the line after it

starts with '+++ '.
It's still possible to break this by changing a single line from '-- ...' to

'++ ...' but that should hopefully be a vanishingly small case, and I suspect

it will also break every other tool that handles diff files.

Manually tested git-svn and git-p4 cases. We don't have any unit tests for

these setups. Ran existing unit tests.