Fix a couple of CommentAsPatch issues. (#14804) (#14820)

Fix a couple of CommentAsPatch issues.  (#14804) (#14820)

Backport #14804

* CutDiffAroundLine makes the incorrect assumption that `---` and `+++` always represent part of the header of a diff.

This PR adds a flag to its parsing to prevent this problem and adds a streaming parsing technique to CutDiffAroundLine using an io.pipe instead of just sending data to an unbounded buffer.

Fix #14711

* Handle unquoted comment patch files

When making comment patches unfortunately the patch does not always quote the filename
This makes the diff --git header ambiguous again.

This PR finally adds handling for ambiguity in to parse patch

Fix #14812

* Add in testing for no error

There is no way currently for CutDiffAroundLine in this test to cause an
error however, it should still be tested.

Signed-off-by: Andrew Thornton <art27@cantab.net>