docs/working-copy: Replace 'del index[path]' with index.remove(path)

Catch up with b12a5960 (Remove "del index[xxx]" from the API,
2013-05-02).
docs/install: Fix 'filling un' -> 'filing a' typo
docs/install: Fix 'instaleld' -> 'installed' typo
Merge remote-tracking branch 'wking/install-filing-typo'
Merge remote-tracking branch 'wking/instaleld-typo'
Refactor code to get the path to libgit2