Fix hyperlink absolute paths

[dandavison]

There were a few bugs causing hyperlinks to have an incorrect target path -- related to the folllowing:

• User not being in repo root dir
• User using git's --relative flag or diff.relative setting
• User using delta's relative-paths option
• Delta invoked by git or input piped to delta

An interesting fact is that when delta is invoked by git, the directory associated with the delta process is the repo root dir, even if delta is started in a deeper dir in the repo (this can be seen by doing git -c core.pager=pwd show).

This PR is an attempt to make everything correct. Here's how delta should behave:

• Absolute paths in hyperlinks must be correct under all circumstances or else a hyperlink must not be displayed
• If delta's relative-paths option is active, all relative paths displayed to the user should be correct from the user's current directory (so that they can be cat'd, clicked on in terminal emulators that support that such as VSCode's, etc)
• The above should be true whether diff.relative is enabled or not