Removing arbitrary refspecs makes things more complex to reason
about. Instead, let the user set the fetch and push refspec list to
whatever they want it to be.

This allows us to add e.g. "HEAD" as a refspec when none are given
without overwriting the user's data.

When downloading the default branch due to lack of refspecs, we still
need to write out FETCH_HEAD with the tip we downloaded, unfortunately
with a format that doesn't match what we already have.

This avoids sending our whole history bit by bit to the remote in cases
where there is no common history, just to give up in the end.

The number comes from the canonical implementation.

The correct behaviour when a remote has no refspecs (e.g. a URL from the
command-line) is to download the remote's HEAD. Let's do that.

This fixes #1261.

Fix examples to make the important stuff more obvious

Warnings for Windows x64 (MSVC) and GCC on Linux

indexer: remove the stream infix

Improve bad URL handling

I'm not too happy about manually inserting < and > but those get
output as html tags otherwise.

Seems that regexp in Mac OS X and Linux were behaving
differently: while in OS X the empty string didn't
match any value, in Linux it was matching all of them,
so the the second fetch refspec was overwritting the
first one, instead of creating a new one.

Using an unmatcheable regular expression solves the
problem (and seems to be portable).

At some moment git_config_delete_entry lost the ability to delete one entry of
a multivar configuration. The moment you had more than one fetch or push
ref spec for a remote you will not be able to save that remote anymore. The
changes in network::remote::remotes::save show that problem.

I needed to create a new git_config_delete_multivar because I was not able to
remove one or several entries of a multivar config with the current API.
Several tries modifying how git_config_set_multivar(..., NULL) behaved were
not successful.

git_config_delete_multivar is very similar to git_config_set_multivar, and
delegates into config_delete_multivar of config_file. This function search
for the cvar_t that will be deleted, storing them in a temporal array, and
rebuilding the linked list. After calling config_write to delete the entries,
the cvar_t stored in the temporal array are freed.

There is a little fix in config_write, it avoids an infinite loop when using
a regular expression (case for the multivars). This error was found by the
test network::remote::remotes::tagopt.

Use gmtime() instead of gmtime_t()

The latter is not available on Windows

protocol: basic support for multi_ack_detailed

This fixes #1703.

This tells the server that we speak it, but we don't make use of its
extra information to determine if there's a better place to stop
negotiating.

In a somewhat-related change, reorder the capabilities so we ask for
them in the same order as git does.

Also take this opportunity to factor out a fairly-indented portion of
the negotiation logic.