Merge branch 'jk/difftool-in-subdir'

"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

diff --cc t/t7800-difftool.sh
Simple merge