[PATCH 1/1] mingw: fix launching of externals from Unicode paths

["Adam Roben via GitGitGadget" <gitgitgadget@gmail.com>]

From: Adam Roben <adam@roben.org>

If Git were installed in a path containing non-ASCII characters,
commands such as `git am` and `git submodule`, which are implemented as
externals, would fail to launch with the following error:

> fatal: 'am' appears to be a git command, but we were not
> able to execute it. Maybe git-am is broken?

This was due to lookup_prog not being Unicode-aware. It was somehow
missed in 85faec9d3a (Win32: Unicode file name support (except dirent),
2012-03-15).

Note that the only problem in this function was calling
`GetFileAttributes()` instead of `GetFileAttributesW()`. The calls to
`access()` were fine because `access()` is a macro which resolves to
`mingw_access()`, which already handles Unicode correctly. But
`lookup_prog()` was changed to use `_waccess()` directly so that we only
convert the path to UTF-16 once.

To make things work correctly, we have to maintain UTF-8 and UTF-16
versions in tandem in `lookup_prog()`.

Signed-off-by: Adam Roben <adam@roben.org>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 compat/mingw.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/compat/mingw.c b/compat/mingw.c
index 8141f77189..9f02403ebf 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -1161,14 +1161,21 @@ static char *lookup_prog(const char *dir, int dirlen, const char *cmd,
 			 int isexe, int exe_only)
 {
 	char path[MAX_PATH];
+	wchar_t wpath[MAX_PATH];
 	snprintf(path, sizeof(path), "%.*s\\%s.exe", dirlen, dir, cmd);
 
-	if (!isexe && access(path, F_OK) == 0)
+	if (xutftowcs_path(wpath, path) < 0)
+		return NULL;
+
+	if (!isexe && _waccess(wpath, F_OK) == 0)
 		return xstrdup(path);
-	path[strlen(path)-4] = '\0';
-	if ((!exe_only || isexe) && access(path, F_OK) == 0)
-		if (!(GetFileAttributes(path) & FILE_ATTRIBUTE_DIRECTORY))
+	wpath[wcslen(wpath)-4] = '\0';
+	if ((!exe_only || isexe) && _waccess(wpath, F_OK) == 0) {
+		if (!(GetFileAttributesW(wpath) & FILE_ATTRIBUTE_DIRECTORY)) {
+			path[strlen(path)-4] = '\0';
 			return xstrdup(path);
+		}
+	}
 	return NULL;
 }
 
-- 
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