diff: Add a callback to notify of diffed files

Fix Windows symlinks

As requested

The callback will be called for each file, just before the `git_delta_t` gets inserted into the diff list.

When the callback:
- returns < 0, the diff process will be aborted
- returns > 0, the delta will not be inserted into the diff list, but the diff process continues
- returns 0, the delta is inserted into the diff list, and the diff process continues

Instead of returning directly the pattern as the return value, I used an
out parameter, because the function also tests if the passed pathspecs
vector is empty. If yes, it considers that the path "matches", but in
that case there is no matched pattern per se.

No bitfields in public headers b/c packing is compiler-specific

removed obsolete reference to api.html

cMakeList: Prevent MSVCR1x0.dll dependency

Deploys the libgit2/libgit2@9041250 fix to RELWITHDEBINFO
and MINSIZEREL build flavors

Fix #255

Handle "user@" prefix for credentials partially included in URLs

revparse: Lookup branch before described tag

Fix #1306

Win32: Make sure error messages are consistently UTF-8 encoded

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Support local msysgit installations

Try harder to find global config file

Signed-off-by: Sven Strickroth <email@cs-ware.de>

W/o this a libgit2 error message could have a mixed encoding:
e.g. a filename in UTF-8 combined with a native Windows error message
encoded with the local code page.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Fix clone fail if repo head detached