Problem:
Using the `DiffTool` plugin (e.g. through `nvim -d ...` or `:DiffTool
<file1> <file2>` fails if a space is in one of the paths. This occurs
because the `diff` wraps the paths with quotes (`'`) if space
characters are present, which the line diff regex fails to parse.
Solution:
Update regex to handle quoted paths by matching the string within the
quotes, if it exists.
(cherry picked from commit 1e09b020e5)
f62ce1a42f
