win32: improve impl & tests for system path / g4w interop

We look for a Git for Windows installation to use its git config,
so that clients built on libgit2 can interoperate with the Git for
Windows CLI (and clients that are built on top of _it_).

Look for `git` both in the `PATH` and in the registry.  Use the _first_
git install in the path, and the first git install in the registry.

Look in both the `etc` dir and the architecture-specific `etc` dirs
(`mingw64/etc` and `mingw32/etc`) beneath the installation root.

Prefer the git in the `PATH` to the git location in the registry so that
users can override that.

Include more tests for this behavior.

Author: ethomson

src/sysdir.c

@@ -31,7 +31,7 @@ static int git_sysdir_guess_programdata_dirs(git_str *out)
 static int git_sysdir_guess_system_dirs(git_str *out)
 {
 #ifdef GIT_WIN32
-	return git_win32__find_system_dirs(out, L"etc\\");
+	return git_win32__find_system_dirs(out, "etc");
 #else
 	return git_str_sets(out, "/etc");
 #endif
@@ -154,7 +154,7 @@ static int git_sysdir_guess_xdg_dirs(git_str *out)
 static int git_sysdir_guess_template_dirs(git_str *out)
 {
 #ifdef GIT_WIN32
-	return git_win32__find_system_dirs(out, L"share\\git-core\\templates");
+	return git_win32__find_system_dirs(out, "share/git-core/templates");
 #else
 	return git_str_sets(out, "/usr/share/git-core/templates");
 #endif
@@ -190,22 +190,22 @@ int git_sysdir_global_init(void)
 		error = git_sysdir__dirs[i].guess(&git_sysdir__dirs[i].buf);
 
 	if (error)
-	    return error;
+		return error;
 
 	return git_runtime_shutdown_register(git_sysdir_global_shutdown);
 }
 
 int git_sysdir_reset(void)
 {
-    size_t i;
-    int error = 0;
+	size_t i;
+	int error = 0;
 
-    for (i = 0; !error && i < ARRAY_SIZE(git_sysdir__dirs); ++i) {
-	git_str_dispose(&git_sysdir__dirs[i].buf);
-	error = git_sysdir__dirs[i].guess(&git_sysdir__dirs[i].buf);
-    }
+	for (i = 0; !error && i < ARRAY_SIZE(git_sysdir__dirs); ++i) {
+		git_str_dispose(&git_sysdir__dirs[i].buf);
+		error = git_sysdir__dirs[i].guess(&git_sysdir__dirs[i].buf);
+	}
 
-    return error;
+	return error;
 }
 
 static int git_sysdir_check_selector(git_sysdir_t which)