On Windows, there are several categories of absolute paths. One such
category starts with a backslash and is implicitly relative to the
drive associated with the current working directory. Example:

	c:
	git clone https://github.com/git-for-windows/git \G4W

should clone into C:\G4W.

There is currently a problem with that, in that mingw_mktemp() does not
expect the _wmktemp() function to prefix the absolute path with the
drive prefix, and as a consequence, the resulting path does not fit into
the originally-passed string buffer. The symptom is a "Result too large"
error.

Reported by Juan Carlos Arevalo Baeza.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

When specifying an absolute path without a drive prefix, we convert that
path internally. Let's make sure that we handle that case properly, too
;-)

This fixes the command

	git clone https://github.com/git-for-windows/git \G4W

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>