odb: move hashing to the frontend for streaming

odb: Straighten oid prefix handling

index: report when it's locked

When implementing the ssh testing, the move to the script made it so
the first test suite's exit code was ignored. Check whether the main
tests fail and exit with an error in that case.

Report the index being locked with its own error code in order to be
able to differentiate, as a locked index is typically the result of a
crashed process or concurrent access, both of which often require user
intervention to fix.

Bring Valgrind back!

If none of the backends support direct writes and we must stream the
whole file, we already know what the object's id should be; so use the
stream's functions directly, bypassing the frontend's hashing and
overwriting of our existing id.

Clarify the role of each function and in particular mention that there
is no need for the backend or stream to worry about the object's id,
as it will be given when `finalize_write` is called.

The frontend is in charge of calculating the id of the objects. Thus
the backends should treat it as a read-only value. The positioning in
the function signature made it seem as though it was an output
parameter.

Make the id const and move it from the front to behind the subject
(backend or stream).

push: handle tag chains correctly

Add "git init"-like example

This adds a new API for creating a signature that uses the
config to look up "user.name" and "user.email".

Give credit to PHP for the p_readlink function in posix_w32.c

When dealing with a chain of tags, we need to enqueue each of them
individually, which means we can't use `git_tag_peel` as that jumps
over the intermediate tags.

Do the peeling manually so we can look at each object and take the
appropriate action.

Hash the data as it's coming into the stream and tell the backend what
its name is when finalizing the write. This makes it consistent with
the way a plain git_odb_write() performs the write.

This is in preparation for moving the hashing to the frontend, which
requires us to handle the incoming data before passing it to the
backend's stream.

revparse: Free left side of invalid range revspecs

This fixes a small memory leak in git_revparse where early returns on
errors from git_revparse_single cause a free() on the (reallocated) left
side of the revspec to be skipped.

That commit accidentally removed the initialization of the "start"
variable giving undefined results for the host extraction from the
url input.

Respect GIT_SSL_NO_VERIFY and http.sslVerify

remote: relax the url rules

Test SSH in travis

fileops: stat() before open()ing in git_futils_readbuffer_updated()

Do not use the "experimental" lookup mode

Accept any value for the remote's url, including an empty string which
we used to reject as invalid configuration.

This is not quite what git does (although it has its own problems with
such configurations) and it makes it harder to fix the issue, by not
letting the user modify it.

As we already need to check for a valid URL when we try to connect to
the network, let that perform the check, as we don't need to do it
anywhere else.

Set up the ssh credentials so we are able to talk to localhost and
issue git commands. Move to use a script, as the command list is
getting somewhat long.

While here, delay installing valgrind until we need it, as it and its
dependencies are by far the largest downloads and this allows us to
start compiling (and failing) faster and we only incur this cost when
the test suite runs successfully.