Built-in merge tools contain a hard-coded assumption about
whether or not a tool's exit code can be trusted to determine
the success or failure of a merge.  Tools whose exit codes are
not trusted contain calls to check_unchanged() in their
merge_cmd() functions.

A problem with this is that the trustExitCode configuration is
not honored for built-in tools.

Teach built-in tools to honor the trustExitCode configuration.
Extend run_merge_cmd() so that it is responsible for calling
check_unchanged() when a tool's exit code cannot be trusted.
Remove check_unchanged() calls from scriptlets since they are no
longer responsible for calling it.

When no configuration is present, exit_code_trustable() is
checked to see whether the exit code should be trusted.
The default implementation returns false.

Tools whose exit codes can be trusted override
exit_code_trustable() to true.

Reported-by: Dun Peal <dunpealer@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Code Compare is a commercial file comparison tool for Windows, see

    http://www.devart.com/codecompare/

Version 2.80.4 added support for command line arguments preceded by a
dash instead of a slash. This is required for Git for Windows because
slashes in command line arguments get mangled with according to these
rules:

    http://www.mingw.org/wiki/Posix_path_conversion

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>