Path used by "git grep" search should be saved per-session

Preamble: I work on QtWebKit, and use several working trees of the same git repo with different branches active, because it's quite inconvenient to switch significantly differnet branches back and forth in such a large repo. I use separate session for each working tree to avoid mixing up source files from different working copies.

Currently the only reliable way to use git grep-based search is using absolute path. However, in my case each session needs to do search inside its own working tree of git repo. Since path is not saved per-session, it's easy to do git grep search in wrong working tree after session switch, and mix up sources from different trees.

Therefore I propose to save search path per-session