git-wrapper: support MSys2

The original purpose of the Git wrapper is to run from inside Git for
Windows' /cmd/ directory, to allow setting up some environment variables
before Git is allowed to take over.

Due to differences in the file system layout, MSys2 requires some
changes for that to work.

In addition, we must take care to set the `MSYSTEM` environment variable
to `MINGW32` or `MINGW64`, respectively, to allow MSys2 to be configured
correctly in case Git launches a shell or Perl script.

We also need to change the `TERM` variable to `cygwin` instead of
`msys`, otherwise the pager `less.exe` (spawned e.g. by `git log`) will
simply crash with a message similar to this one:

	1 [main] less 9832 cygwin_exception::open_stackdumpfile:
	Dumping stack trace to less.exe.stackdump

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

Author: dscho

compat/win32/git-wrapper.c

@@ -12,6 +12,9 @@
 #include <shlwapi.h>
 #include <shellapi.h>
 #include <stdio.h>
+#include <wchar.h>
+
+static WCHAR msystem_bin[64];
 
 static void print_error(LPCWSTR prefix, DWORD error_number)
 {
@@ -36,12 +39,18 @@ static void print_error(LPCWSTR prefix, DWORD error_number)
 
 static void setup_environment(LPWSTR exepath)
 {
-	int len;
+	WCHAR msystem[64];
 	LPWSTR path2 = NULL;
+	int len;
 
-	/* if not set, set TERM to msys */
+	/* Set MSYSTEM */
+	swprintf(msystem, sizeof(msystem),
+		L"MINGW%d", (int) sizeof(void *) * 8);
+	SetEnvironmentVariable(L"MSYSTEM", msystem);
+
+	/* if not set, set TERM to cygwin */
 	if (!GetEnvironmentVariable(L"TERM", NULL, 0))
-		SetEnvironmentVariable(L"TERM", L"msys");
+		SetEnvironmentVariable(L"TERM", L"cygwin");
 
 	/* if not set, set PLINK_PROTOCOL to ssh */
 	if (!GetEnvironmentVariable(L"PLINK_PROTOCOL", NULL, 0))
@@ -79,12 +88,22 @@ static void setup_environment(LPWSTR exepath)
 	len = sizeof(WCHAR) * (len + 2 * MAX_PATH);
 	path2 = (LPWSTR)malloc(len);
 	wcscpy(path2, exepath);
-	PathAppend(path2, L"bin;");
-	/* should do this only if it exists */
-	wcscat(path2, exepath);
-	PathAppend(path2, L"mingw\\bin;");
-	GetEnvironmentVariable(L"PATH", &path2[wcslen(path2)],
-			(len/sizeof(WCHAR))-wcslen(path2));
+	PathAppend(path2, msystem_bin);
+	if (_waccess(path2, 0) != -1) {
+		/* We are in an MSys2-based setup */
+		wcscat(path2, L";");
+		wcscat(path2, exepath);
+		PathAppend(path2, L"usr\\bin;");
+	}
+	else {
+		/* Fall back to MSys1 paths */
+		wcscpy(path2, exepath);
+		PathAppend(path2, L"bin;");
+		wcscat(path2, exepath);
+		PathAppend(path2, L"mingw\\bin;");
+	}
+	GetEnvironmentVariable(L"PATH", path2 + wcslen(path2),
+				(len / sizeof(WCHAR)) - wcslen(path2));
 	SetEnvironmentVariable(L"PATH", path2);
 	free(path2);
 
@@ -156,6 +175,10 @@ int main(void)
 	LPWSTR cmd = NULL, exep = exe, prefix_args = NULL, basename;
 	UINT codepage = 0;
 
+	/* Determine MSys2-based Git path. */
+	swprintf(msystem_bin, sizeof(msystem_bin),
+		L"mingw%d\\bin", (int) sizeof(void *) * 8);
+
 	/* get the installation location */
 	GetModuleFileName(NULL, exepath, MAX_PATH);
 	if (!PathRemoveFileSpec(exepath)) {
@@ -183,7 +206,12 @@ int main(void)
 
 		/* set the default exe module */
 		wcscpy(exe, exepath);
-		PathAppend(exe, L"bin\\git.exe");
+		PathAppend(exe, msystem_bin);
+		PathAppend(exe, L"git.exe");
+		if (_waccess(exe, 0) == -1) {
+			wcscpy(exe, exepath);
+			PathAppend(exe, L"bin\\git.exe");
+		}
 	}
 
 	if (!prefix_args)