Merge branch 'jk/difftool-in-subdir' into maint

"git difftool <paths>..." started in a subdirectory failed to interpret the paths relative to that directory, which has been fixed. * jk/difftool-in-subdir: difftool: use Git::* functions instead of passing around state difftool: avoid $GIT_DIR and $GIT_WORK_TREE difftool: fix argument handling in subdirs