Adds functions for accessing git index content and working tree files:
- get_index_content() retrieves file from staging area via :0:path
- get_working_content() reads file directly from disk
- file_exists_in_index() checks if file is staged
- file_exists_at_revision() checks if file exists at given revision
Compares current buffer against any git revision (default HEAD), opens result
with full diffs.nvim syntax highlighting. Follows fugitive convention:
:Gdiff/:Gvdiff open vertical split, :Ghdiff opens horizontal split.
