Check for a git_binary global preference

Just like GitGutter.
Add git_goto_diff command to jump to the diff line with Enter key
Store git_working_dir in the view settings instead of using a global var
Use git root directory to create absolute path names in GitGotoDiff

Problem reported by @vanrijn (Jason Kasper)
Adding double-click mousemap file so we can navigate git diff buffer that way too.
removed mousemap
Fix incorrect assumption that git_root_dir is populated and allow GitGotoDiff to work with Git Custom commands
Sanity-checking git_root_dir and if it's not valid for the current hunk's file, prompt the user to fix it rather than just failing.
Stripping whitespace off filename to fix problem with filename with spaces.
Open all diffs in diff_panel
Split optimizations
Added --follow to diffs and added revision numbers
Fixed marking of added/removed lines
Do not --follow for Log All
Remove added regions for inserts/deletes (doesn't always work)
Fixed some commands. Simplified blame output
Added missing import
Change how PLUGIN_DIRECTORY is generated
Remove debug message
Python3 compatibility