Git wrapper: allow overriding what executable is called

The Git wrapper does one thing, and does it well: setting up the
environment required to run Git and its scripts, and then hand off to
another program.

We already do this for the Git executable itself; in Git for Windows'
context, we have exactly the same need also when calling the Git Bash or
Git CMD. However, both are tied to what particular shell environment you
use, though: MSys or MSys2 (or whatever else cunning developers make
work for them). This means that the Git Bash and Git CMD need to be
compiled in the respective context (e.g. when compiling the
mingw-w64-git package in the MSys2 context).

Happily, Windows offers a way to configure compiled executables:
resources. So let's just look whether the current executable has a
string resource and use it as the command-line to execute after the
environment is set up. To support MSys2's Git Bash better (where
`mintty` should, but might not, be available), we verify whether the
specified executable exists, and keep looking for string resources if it
does not.

For even more flexibility, we expand environment variables specified as
`@@<VARIABLE-NAME>@@`, and for convenience `@@EXEPATH@@` expands into
the directory in which the executable resides.

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

compat/win32/git-wrapper.c

@@ -173,6 +173,102 @@ static LPWSTR fixup_commandline(LPWSTR exepath, LPWSTR *exep, int *wait,
 	return cmd;
 }
 
+static int configure_via_resource(LPWSTR basename, LPWSTR exepath, LPWSTR exep,
+	LPWSTR *prefix_args, int *prefix_args_len,
+	int *is_git_command, int *start_in_home)
+{
+	int id, wargc;
+	LPWSTR *wargv;
+
+#define BUFSIZE 65536
+	static WCHAR buf[BUFSIZE];
+	int len;
+
+	for (id = 0; ; id++) {
+		len = LoadString(NULL, id, buf, BUFSIZE);
+
+		if (!len) {
+			if (!id)
+				return 0; /* no resources found */
+
+			fwprintf(stderr, L"Need a valid command-line; "
+				L"Edit the string resources accordingly\n");
+			exit(1);
+		}
+
+		if (len >= BUFSIZE) {
+			fwprintf(stderr,
+				L"Could not read resource (too large)\n");
+			exit(1);
+		}
+
+		buf[len] = L'\0';
+
+		if (!id)
+			SetEnvironmentVariable(L"EXEPATH", exepath);
+
+		for (;;) {
+			LPWSTR atat = wcsstr(buf, L"@@"), atat2;
+			WCHAR save;
+			int env_len, delta;
+
+			if (!atat)
+				break;
+
+			atat2 = wcsstr(atat + 2, L"@@");
+			if (!atat2)
+				break;
+
+			*atat2 = L'\0';
+			env_len = GetEnvironmentVariable(atat + 2, NULL, 0);
+			delta = env_len - 1 - (atat2 + 2 - atat);
+			if (len + delta >= BUFSIZE) {
+				fwprintf(stderr,
+					L"Substituting '%s' results in too "
+					L"large a command-line\n", atat + 2);
+				exit(1);
+			}
+			if (delta)
+				memmove(atat2 + 2 + delta, atat2 + 2,
+					sizeof(WCHAR) * (len + 1
+						- (atat2 + 2 - buf)));
+			len += delta;
+			save = atat[env_len - 1];
+			GetEnvironmentVariable(atat + 2, atat, env_len);
+			atat[env_len - 1] = save;
+		}
+
+		/* parse first argument */
+		wargv = CommandLineToArgvW(buf, &wargc);
+		if (wargc < 1) {
+			fwprintf(stderr, L"Invalid command-line: '%s'\n", buf);
+			exit(1);
+		}
+		if (*wargv[0] == L'\\' ||
+				(isalpha(*wargv[0]) && wargv[0][1] == L':'))
+			wcscpy(exep, wargv[0]);
+		else {
+			wcscpy(exep, exepath);
+			PathAppend(exep, wargv[0]);
+		}
+		LocalFree(wargv);
+
+		if (_waccess(exep, 0) != -1)
+			break;
+		fwprintf(stderr,
+			L"Skipping command-line '%s'\n('%s' not found)\n",
+			buf, exep);
+	}
+
+	*prefix_args = buf;
+	*prefix_args_len = wcslen(buf);
+
+	*is_git_command = 0;
+	*start_in_home = 1;
+
+	return 1;
+}
+
 int main(void)
 {
 	int r = 1, wait = 1, prefix_args_len = -1, needs_env_setup = 1,
@@ -192,7 +288,12 @@ int main(void)
 		ExitProcess(1);
 	}
 	basename = exepath + wcslen(exepath) + 1;
-	if (!wcsncmp(basename, L"git-", 4)) {
+	if (configure_via_resource(basename, exepath, exep,
+			&prefix_args, &prefix_args_len,
+			&is_git_command, &start_in_home)) {
+		/* do nothing */
+	}
+	else if (!wcsncmp(basename, L"git-", 4)) {
 		needs_env_setup = 0;
 
 		/* Call a builtin */