mingw: rework the environment handling

Once upon a time, the Git for Windows project had to decide what to do about Unicode support, including how to deal with the environment. Karsten Blees spent a ton of work on this, culminating in the final version
[https://groups.google.com/d/msg/msysgit/wNZAyScbJG4/viWz2KXU0VYJ] which made it into Git for Windows and at least partially into core Git, too.

The environment handling in particular is a bit tricky: Windows actually has two copies of the environment, one encoded in UTF-16, and the other one in the local encoding. Since we want UTF-8 encoded values (which is not an option for the local encoding), we had to convert from/to the UTF-16 environment.

At the time those patches were developed, there were so many getenv()/ putenv() calls in Git's code base that it seemed the best solution to convert the entire environment into UTF-8 in one go, at startup.

There are good reasons for us to change that paradigm now (and this patch series does that):

* The method we use does not work with modern MSVC runtimes (__environ can no longer be overridden).
* Our method of having a malloc()ed environment wreaks havoc if a library we use calls MSVC's version of setenv() (I am looking at you, libcurl).
* In the meantime, core Git's usage of getenv()/putenv() was reduced dramatically (for unrelated reasons), so that it is actually advantageous nowadays to convert on the fly, i.e. with each getenv()/putenv() call, rather than doing one wholesale conversion at process startup. See also the commit message of the second patch.

Note: in contrast to other patches flowing from Git for Windows to Git these days this patch has not been in Git for Windows for ages. Its approach has been tested in some MS Visual C++ builds (thanks, Jeff Hostetler!), though, so I am quite confident that it is correct, and the test suite agrees.

Johannes Schindelin (2):
t7800: fix quoting
mingw: reencode environment variables on the fly (UTF-16 <-> UTF-8)

 compat/mingw.c      | 280 ++++++++++++++++++++++++++------------------
 compat/mingw.h      |  32 ++++-
 t/t7800-difftool.sh |   2 +-
 3 files changed, 197 insertions(+), 117 deletions(-)

base-commit: c670b1f876521c9f7cd40184bf7ed05aad843433
Published-As: https://github.com/gitgitgadget/git/releases/tags/pr-57%2Fdscho%2Fmingw-utf-8-env-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-57/dscho/mingw-utf-8-env-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/57

gitgitgadget