mergetool--lib: simplify API usage by removing more global variables

The mergetool--lib scriplet was tricky to use because it relied upon
the existance of several global shell variables.  This removes more
global variables so that things are simpler for callers.

A side effect is that some variables are recomputed each time
run_merge_tool() is called, but the overhead for recomputing
them is justified by the simpler implementation.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-mergetool.sh

@@ -174,9 +174,11 @@ merge_file () {
 	read ans
     fi
 
-    present=false
-    base_present &&
-    present=true
+    if base_present; then
+	    present=true
+    else
+	    present=false
+    fi
 
     if ! run_merge_tool "$merge_tool" "$present"; then
 	echo "merge of $MERGED failed" 1>&2
@@ -254,12 +256,11 @@ prompt_after_failed_merge() {
     done
 }
 
-merge_tool=$(get_merge_tool "$merge_tool") || exit
-merge_tool_cmd="$(get_merge_tool_cmd "$merge_tool")"
-merge_tool_path="$(get_merge_tool_path "$merge_tool")" || exit
+if test -z "$merge_tool"; then
+    merge_tool=$(get_merge_tool "$merge_tool") || exit
+fi
 merge_keep_backup="$(git config --bool mergetool.keepBackup || echo true)"
 merge_keep_temporaries="$(git config --bool mergetool.keepTemporaries || echo false)"
-merge_tool_trust_exit_code="$(git config --bool mergetool."$merge_tool".trustExitCode || echo false)"
 
 last_status=0
 rollup_status=0