improve index mode for executable files

The fix for gitpython-developers#430 in bebc4f5 (Use correct mode for executable files,
2016-05-19) is incomplete.  It fails (in most cases) when files have
modes which are not exactly 0644 or 0755.

As git only cares about executable or or not in this case, ensure the
mode we set for the index is either 100644 or 100755.  Do this similarly
to how upstream git does it in cache.h¹.

Fixes gitpython-developers#1253

¹ https://github.com/git/git/blob/v2.31.1/cache.h#L247

Author: tmzullinger

git/index/fun.py

@@ -10,6 +10,7 @@
     S_ISDIR,
     S_IFMT,
     S_IFREG,
+    S_IXUSR,
 )
 import subprocess
 
@@ -103,7 +104,7 @@ def stat_mode_to_index_mode(mode):
         return S_IFLNK
     if S_ISDIR(mode) or S_IFMT(mode) == S_IFGITLINK:    # submodules
         return S_IFGITLINK
-    return S_IFREG | 0o644 | (mode & 0o111)       # blobs with or without executable bit
+    return S_IFREG | (mode & S_IXUSR and 0o755 or 0o644) # blobs with or without executable bit
 
 
 def write_cache(entries, stream, extension_data=None, ShaStreamCls=IndexFileSHA1Writer):