mergetool: break setup_tool out into separate initialization function

This is preparation for the following commit where we need to source the
mergetool shell script to look for overrides before `run_merge_tool` is
called. Previously `run_merge_tool` both sourced that script and invoked
the mergetool.

In the case of the following commit, we need the result of the
`hide_resolved` override, if present, before we actually run
`run_merge_tool`.

The new `initialize_merge_tool` wrapper is exposed and documented as
a public interface for consistency with the existing `run_merge_tool`
which is also public. Although `setup_tool` could instead be exposed
directly, the related `setup_user_tool` would probably also want to be
elevated to match and this felt the cleanest to me.

Signed-off-by: Seth House <seth@eseth.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-difftool--helper.sh

@@ -61,6 +61,9 @@ launch_merge_tool () {
 		export BASE
 		eval $GIT_DIFFTOOL_EXTCMD '"$LOCAL"' '"$REMOTE"'
 	else
+		initialize_merge_tool "$merge_tool"
+		# ignore the error from the above --- run_merge_tool
+		# will diagnose unusable tool by itself
 		run_merge_tool "$merge_tool"
 	fi
 }
@@ -79,6 +82,9 @@ if test -n "$GIT_DIFFTOOL_DIRDIFF"
 then
 	LOCAL="$1"
 	REMOTE="$2"
+	initialize_merge_tool "$merge_tool"
+	# ignore the error from the above --- run_merge_tool
+	# will diagnose unusable tool by itself
 	run_merge_tool "$merge_tool" false
 else
 	# Launch the merge tool on each path provided by 'git diff'