Fix blame and status to work from within directories starting with a dot

When running `tig blame ../io.h' or `tig status' inside the directory
`.dir', the command:

	git rev-parse --git-dir --is-inside-work-tree --show-cdup --show-prefix

will print:

	/tig/.git
	true
	../
	.dir/

This cause read_repo_info() to confuse the show-prefix option output was
the output of the show-cdup option.

Fixes #173 reported by Azat Khuzhin.