repository: Refine repository_head() error report

The indexer handles this better than the fetch
logic does.

create FETCH_HEAD specially instead of as a ref file

Duplicate tree entries

Topic/explicit index errors

Fixes for two segfaults

config: make git_config_open_level() work with an empty config

Fix libgit2/libgit2sharp#243

Valgrind reports Invalid Read when the configuration file contains some empty line

Update documentation in remote.h

Basic authentication for http and winhttp

- Update 'tests-clar/resources/config/config11' in order to reproduce the invalidread with the unittest (just added some \n at the end of the file)
- Fix config_file.c

Correctly parse tags lacking a description

Fix compilation for mingw32 and cygwin

inet_pton is available only in windows vista or later,
fixed the issue by reimplementing it using WSAStringToAddress

fetch: Write packs to the ODB instead of directly to disk