List a submodule only once when the path matches a submodule in the index

examples: modernise the fetch example

The header src/cc-compat.h defines portable format specifiers PRIuZ, PRIdZ, and PRIxZ. The original report highlighted the need to use these specifiers in examples/network/fetch.c.  For this commit, I checked all C source and header files not in deps/ and transitioned to the appropriate format specifier where appropriate.

Under normal conditions, git_remote_fetch() should be the only function
used to perform a fetch. Don't let the example lead people astray.

Increase required version of cmake to 2.8

If we get the path from the gitmodules file, look up the submodule we're
interested in by path, rather then by name. Otherwise we might get
duplicate results.

When we rename a submodule, we should be merging two sets of information
based on whether their path is the same. We currently only deduplicate
on equal name, which causes us to double-report.

Clean up some warnings

Fix undefined reference with old versions of openssl

git_cert: child types use proper base type

filters: custom filters with wildcard attributes

Don't add unnecessary trailing newline during file merge

Upgrade xdiff to version used in core git 2.4.5 (0df0541).

Corrects an issue where an LF is added at EOF while applying
an unrelated change (ba31180), cleans up some unused code (be89977 and
e5b0662), and provides an improved callback to avoid leaking internal
(to xdiff) structures (467d348).

This also adds some additional functionality that we do not yet take
advantage of, namely the ability to ignore changes whose lines are
all blank (36617af).

When invoked with three files that each lack a trailing newline,
the merge result should also lack a trailing newline.

meta: Add Antoine Pelisse to hall-of-fame list

Antoine Pelisse <apelisse@gmail.com> has kindly allowed his
contributions to core git to be used under the libgit2 license.

Versions prior to 0.9.8f  did not have this function, rhel/centos5 are still on a
heavily backported version of 0.9.8e and theoretically supported until March 2017

Without this ifdef, I get the following link failure:
```
CMakeFiles/libgit2_clar.dir/src/openssl_stream.c.o: In function `openssl_connect':
openssl_stream.c:(.text+0x45a): undefined reference to `SSL_set_tlsext_host_name'
collect2: error: ld returned 1 exit status
make[6]: *** [libgit2_clar] Error 1
```

Added git_diff_index_to_index()

git__getenv: utf-8 aware env reader

Stacktraces with CRTDBG memory leaks on Windows

Homebrew will error out because it's already installed.

We need to make sure we are asking for the current version of packages,
or we might get 404s from the download service.

filter: add docs for streaming filters

These functions are available on the public API but don't have any
documentation, so they don't appear on the API reference. Fix that.

iterator_walk: cast away constness for free

Introduce `git__getenv` which is a UTF-8 aware `getenv` everywhere.
Make `cl_getenv` use this to keep consistent memory handling around
return values (free everywhere, as opposed to only some platforms).

filter::stream: free the filter sanely

Don't use the filter's free callback to free the actual data structure
holding the filter, as we may not always actually initialize it (the
test may be skipped).

Correctly delimit the keys for submodule lookup

The regex we use to look at the gitmodules file does not correctly
delimit the name of submodule which we want to look up and puts '.*'
straight after the name, maching on any submodule which has the seeked
submodule as a prefix of its name.

Add the missing '\.' in the regex so we want a full stop to exist both
before and after the submodule name.