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: NDun Peal <dunpealer@gmail.com>
Signed-off-by: NDavid Aguilar <davvid@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Individual merge tools are now defined in a mergetools/$tool
file which is sourced at runtime.

The individual files are installed into $(git --exec-path)/mergetools/.
New tools can be added by creating a new file instead of editing the
git-mergetool--lib.sh scriptlet.

http://thread.gmane.org/gmane.comp.version-control.git/134906/focus=135006Signed-off-by: NDavid Aguilar <davvid@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>