Change the default shortcut for git blame on Linux (#13637)

Zed already has a shortcut assigned to ctrl-alt-g and it's mapped to
`search::SelectNextMatch`. Having another multi shortcut with the same
prefix makes `ctrl-alt-g` to have a very noticeable delay when pressed.

This commit changes the default shortcut for git blame to `alt-g b`

Release Notes:

- N/A