git: fix launching the Git wrapper from network drives

Since 521a655 (git: fix 'cmd/git.exe' cannot launcher symlink,
2018-12-08), we always try to resolve the Git wrapper's executable path
if it is a symbolic link. We do that by calling GetFinalPathNByHandle(),
but that introduced a regression where we would not handle UNC paths
correctly.

This fixes git-for-windows/git#2036

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

mingw-w64-git/git-wrapper.c

@@ -64,8 +64,19 @@ static void find_exe_realpath(LPWSTR exepath, int count) {
  if (dwlen >= MAX_PATH) {
  wcscpy(exepath, mdexe);
  } else {
- size_t offset = wcsncmp(L"\\\\?\\", realexe, 4) ? 0 : 4;
- size_t length = dwlen - offset;
+ size_t offset = 0;
+ size_t length = dwlen;
+
+ if (!wcsncmp(realexe, L"\\\\?\\", 4)) {
+ if (wcsncmp(realexe + 4, L"UNC\\", 4))
+ offset = 4;
+ else {
+ offset = 7;
+ *(exepath++) = L'\\';
+ }
+ length -= offset;
+ }
+
  wcsncpy(exepath, realexe + offset, length);
  exepath[length] = L'\0';
  }