Git.pm: stop assuming that absolute paths start with a slash

Git.pm: stop assuming that absolute paths start with a slash

On Windows, absolute paths never start with a slash, unless a POSIX
emulation layer is used. The latter is the case for MSYS2's Perl that
Git for Windows leverages. However, in the tests we also go through
plain `git.exe`, which does *not* leverage the POSIX emulation layer,
and therefore the paths we pass to Perl may actually be DOS-style paths
such as C:/Program Files/Git.

So let's just use Perl's own way to test whether a given path is
absolute or not instead of home-brewing our own.

This patch partially fixes t7800 and t9700 when running in Git for
Windows' SDK.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>