mergetool: honor mergetool.$tool.trustExitCode for built-in tools

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>

mergetools/vimdiff

@@ -4,7 +4,6 @@ diff_cmd () {
 }
 
 merge_cmd () {
-	touch "$BACKUP"
 	case "$1" in
 	gvimdiff|vimdiff)
 		if $base_present
@@ -31,7 +30,6 @@ merge_cmd () {
 		fi
 		;;
 	esac
-	check_unchanged
 }
 
 translate_merge_tool_path() {