patch 8.0.0645: no error for illegal back reference in NFA engine

Problem:    The new regexp engine does not give an error for using a back
            reference where it is not allowed. (Dominique Pelle)
Solution:   Check the back reference like the old engine. (closes vim#1774)

patch 8.0.0644: the timeout for 'hlsearch' is not tested

Problem:    There is no test for 'hlsearch' timing out.
Solution:   Add a test.

patch 8.0.0643: when a pattern search is slow Vim becomes unusable

Problem:    When 'hlsearch' is set and matching with the last search pattern
            is very slow, Vim becomes unusable.  Cannot quit search by
            pressing CTRL-C.
Solution:   When the search times out set a flag and don't try again.  Check
            for timeout and CTRL-C in NFA loop that adds states.

patch 8.0.0642: writefile() continues after detecting an error

Problem:    writefile() continues after detecting an error.
Solution:   Bail out as soon as an error is detected. (suggestions by Nikolai
            Pavlov, closes vim#1476)

patch 8.0.0641: cannot set a separate highlighting for the quickfix line

Problem:    Cannot set a separate highlighting for the current line in the
            quickfix window.
Solution:   Add QuickFixLine. (anishsane, closes vim#1755)

patch 8.0.0640: mismatch between help and actual message

Problem:    Mismatch between help and actual message for ":syn conceal".
Solution:   Change the message to match the help. (Ken Takata)

patch 8.0.0639: the cursor position is set when editing a new commit …

…message

Problem:    The cursor position is set to the last position in a new commit
            message.
Solution:   Don't set the position if the filetype matches "commit".
            (Christian Brabandt)

patch 8.0.0638: cannot build with new MSVC version

Problem:    Cannot build with new MSVC version VS2017.
Solution:   Change the compiler arguments. (Leonardo Manera, closes vim#1731,
            closes vim#1747)

patch 8.0.0637: crash when using some version of GTK 3

Problem:    Crash when using some version of GTK 3.
Solution:   Add #ifdefs around incrementing the menu index. (Kazunobu
            Kuriyama)

patch 8.0.0636: when reading the undo file fails may use uninitialize…

…d data

Problem:    When reading the undo file fails may use uninitialized data.
Solution:   Always clear the buffer on failure.