BaseTool: Fixed incremental rebuild issue.

BaseTool: Fixed incremental rebuild issue.

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=1540

This issue in introduced by commit
d943b0c339fe3d35ffdf9f580ccb7a55915c6854

To convert bytes to string, we need to use bytes.decode()
instead of using str(bytes).

If the source file is not a txt file, ignore that file.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>