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>

Documentation/git-mergetool--lib.txt

@@ -38,6 +38,10 @@ get_merge_tool_cmd::
 get_merge_tool_path::
 	returns the custom path for a merge tool.
 
+initialize_merge_tool::
+	bring merge tool specific functions into scope so they can be used or
+	overridden.
+
 run_merge_tool::
 	launches a merge tool given the tool name and a true/false
 	flag to indicate whether a merge base is present.

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'

git-mergetool--lib.sh

@@ -248,6 +248,10 @@ trust_exit_code () {
 	fi
 }
 
+initialize_merge_tool () {
+	# Bring tool-specific functions into scope
+	setup_tool "$1" || return 1
+}
 
 # Entry point for running tools
 run_merge_tool () {
@@ -259,9 +263,6 @@ run_merge_tool () {
 	merge_tool_path=$(get_merge_tool_path "$1") || exit
 	base_present="$2"
 
-	# Bring tool-specific functions into scope
-	setup_tool "$1" || return 1
-
 	if merge_mode
 	then
 		run_merge_cmd "$1"

git-mergetool.sh

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