FS_POSIX: Rework umask detection to make it settable

FS_POSIX: Rework umask detection to make it settable

Avoid always calling `sh -c umask` on startup, instead deferring
the invocation until the first time a working tree file needs to
use the execute bit. This allows servers using bare repos to avoid
a costly fork+exec for a value that is never used.

Store the umask as an int instead of two Boolean. This is slightly
smaller memory (one int vs. two references) and makes it easier for
an application to force setting the umask to a value that overrides
whatever the shell told JGit.

Simplify the code to bail by returning early when canExecute is
false, which is the common case for working tree files.

Change-Id: Ie713647615bc5bdf5d71b731a6748c28ea21c900