mergetool: fix emerge when running in a subdirectory

Only pass the basename of the output filename when to emerge, since
emerge interprets non-absolute pathnames relative to the containing
directory of the output buffer.

Thanks to Kelvie Wong for pointing this out.
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

git-mergetool.sh

@@ -252,9 +252,9 @@ merge_file () {
 	    ;;
 	emerge)
 	    if base_present ; then
-		emacs -f emerge-files-with-ancestor-command "$LOCAL" "$REMOTE" "$BASE" "$path"
+		emacs -f emerge-files-with-ancestor-command "$LOCAL" "$REMOTE" "$BASE" "$(basename "$path")"
 	    else
-		emacs -f emerge-files-command "$LOCAL" "$REMOTE" "$path"
+		emacs -f emerge-files-command "$LOCAL" "$REMOTE" "$(basename "$path")"
 	    fi
 	    status=$?
 	    save_backup