treebuilder: rename _create() to _new()

This function is a constructor, so let's name it like one and leave
_create() for the reference functions, which do create/write the
reference.
Added AmigaOS-specific implementation of git__timer().

The clock_gettime() function is normally not available under
AmigaOS, hence another solution is required. We are using now
GetUpTime() that is present in current versions of this
operating system.
Merge pull request #2777 from sba1/amigaos-get-timer

Added git__timer() variant for AmigaOS.
Merge pull request #2783 from libgit2/cmn/treebuilder-new

treebuilder: rename _create() to _new()