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-mergetool.sh

@@ -272,6 +272,8 @@ merge_file () {
 		ext=
 	esac
 
+	initialize_merge_tool "$merge_tool" || return
+
 	mergetool_tmpdir_init
 
 	if test "$MERGETOOL_TMPDIR" != "."