CI: improve gcov handling #10404

- Move __gcov_flush to process_spawn, for more reliable coverage
  tracking of subprocesses
- Travis: use GCOV_ERROR_FILE
- codecov: use "-X fix" to skip "fixing" uploaded coverage data; it
  should be handled by codecov's backend instead.
- AppVeyor: no $PATH mangling, which breaks with the improved coverage tracking
  due to missing .dll in PATH.

src/nvim/os/pty_process_unix.c

@@ -36,11 +36,6 @@
 # include "os/pty_process_unix.c.generated.h"
 #endif
 
-/// Externally defined with gcov.
-#ifdef USE_GCOV
-void __gcov_flush(void);
-#endif
-
 /// termios saved at startup (for TUI) or initialized by pty_process_spawn().
 static struct termios termios_default;
 
@@ -64,11 +59,6 @@ int pty_process_spawn(PtyProcess *ptyproc)
     init_termios(&termios_default);
   }
 
-#ifdef USE_GCOV
-  // Flush coverage data before forking, to avoid "Merge mismatch" errors.
-  __gcov_flush();
-#endif
-
   int status = 0;  // zero or negative error code (libuv convention)
   Process *proc = (Process *)ptyproc;
   assert(proc->err.closed);