gitk: Show modified files with separate work tree

gitk: Show modified files with separate work tree

"git rev-parse --is-inside-work-tree" is currently used to determine
whether to show modified files in gitk (the red and green fake
commits). This does not work if the current directory is not inside
the work tree, as can be the case e.g. if GIT_WORK_TREE is
set. Instead, check if the repository is not bare and that we are not
inside the .git directory.

Signed-off-by: Martin von Zweigbergk <redacted>
Signed-off-by: Paul Mackerras <redacted>