send-email: recognize absolute path on Windows

On Windows, absolute paths might start with a DOS drive prefix,
which these two checks failed to recognize.

Unfortunately, we cannot simply use the file_name_is_absolute
helper in File::Spec::Functions, because Git for Windows has an
MSYS-based Perl, where this helper doesn't grok DOS
drive-prefixes.

So let's manually check for these in that case, and fall back to
the File::Spec-helper on other platforms (e.g Win32 with native
Perl)

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>