tests: explicitly use `git.exe` on Windows

On Windows, when we refer to `/an/absolute/path/to/git`, it magically resolves `git.exe` at that location. Except if something of the name `git` exists next to that `git.exe`. So if we call `$BUILD_DIR/git`, it will find `$BUILD_DIR/git.exe` *only* if there is not, say, a directory called `$BUILD_DIR/git`. Such a directory, however, exists in Git for Windows when building with Visual Studio (our Visual Studio project generator defaults to putting the build files into a directory whose name is the base name of the corresponding `.exe`). In the bin-wrappers/* scripts, we already take pains to use `git.exe` rather than `git`, as this could pick up the wrong thing on Windows (i.e. if there exists a `git` file or directory in the build directory). Now we do the same in the tests' start-up code. This also helps when testing an installed Git, as there might be even more likely some stray file or directory in the way. Note: the only way we can record whether the `.exe` suffix is by writing it to the `GIT-BUILD-OPTIONS` file and sourcing it at the beginning of `t/test-lib.sh`. This is not a requirement introduced by this patch, but we move the call to be able to use the `$X` variable that holds the file extension, if any. Note also: the many, many calls to `git this` and `git that` are unaffected, as the regular PATH search will find the `.exe` files on Windows (and not be confused by a directory of the name `git` that is in one of the directories listed in the `PATH` variable), while `/path/to/git` would not, per se, know that it is looking for an executable and happily prefer such a directory. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>