mingw: Prepare the TMP environment variable for shell scripts

When shell scripts access a $TMP variable containing backslashes, they
will be mistaken for escape characters. Let's not let that happen by
converting them to forward slashes.

This fixes t7800 with MSys2.

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

compat/mingw.c

@@ -2226,6 +2226,18 @@ void mingw_startup()
 			environ[i] = xstrdup(to_free + 6);
 			free(to_free);
 		}
+		if (!strncasecmp(environ[i], "TMP=", 4)) {
+			/*
+			 * Convert all dir separators to forward slashes,
+			 * to help shell commands called from the Git
+			 * executable (by not mistaking the dir separators
+			 * for escape characters).
+			 */
+			char *p;
+			for (p = environ[i]; *p; p++)
+				if (*p == '\\')
+					*p = '/';
+		}
 	}
 	environ[i] = NULL;
 	free(buffer);