ObjectChecker: Disallow names potentially mapping to ".git" on HFS+

ObjectChecker: Disallow names potentially mapping to ".git" on HFS+

Mac's HFS+ folds concatentations of ".git" and ignorable Unicode
characters [1] to ".git" [2]. Hence we need to disallow all names which
could potentially be a shortname for ".git". Example: in an empty
directory create a folder ".g\U+200Cit". Now you can't create another
folder ".git".

The following characters are ignorable Unicode which are ignored on
HFS+:

unicode   hex         name
-------------------------------------------------
U+200C    0xe2808c    ZERO WIDTH NON-JOINER
U+200D    0xe2808d    ZERO WIDTH JOINER
U+200E    0xe2808e    LEFT-TO-RIGHT MARK
U+200F    0xe2808f    RIGHT-TO-LEFT MARK
U+202A    0xe280aa    LEFT-TO-RIGHT EMBEDDING
U+202B    0xe280ab    RIGHT-TO-LEFT EMBEDDING
U+202C    0xe280ac    POP DIRECTIONAL FORMATTING
U+202D    0xe280ad    LEFT-TO-RIGHT OVERRIDE
U+202E    0xe280ae    RIGHT-TO-LEFT OVERRIDE
U+206A    0xe281aa    INHIBIT SYMMETRIC SWAPPING
U+206B    0xe281ab    ACTIVATE SYMMETRIC SWAPPING
U+206C    0xe281ac    INHIBIT ARABIC FORM SHAPING
U+206D    0xe281ad    ACTIVATE ARABIC FORM SHAPING
U+206E    0xe281ae    NATIONAL DIGIT SHAPES
U+206F    0xe281af    NOMINAL DIGIT SHAPES
U+FEFF    0xefbbbf    ZERO WIDTH NO-BREAK SPACE

[1] http://www.unicode.org/versions/Unicode7.0.0/ch05.pdf#G40025
    http://www.unicode.org/reports/tr31/#Layout_and_Format_Control_Characters
[2] http://dubeiko.com/development/FileSystems/HFSPLUS/tn1150.html#UnicodeSubtleties

Change-Id: Ib6a1dd090b2649bdd8ec16387c994ed29de2860d
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>