Side-step MSYS-specific path "corruption" leading to t5560 failure.

Side-step MSYS-specific path "corruption" leading to t5560 failure.

Upon program invocation, MSYS converts environment variables containing
path-like values from Unix-style to DOS-style under the assumption that
the program being invoked understands only DOS-style pathnames.  For
instance, the Unix-style path /msysgit is translated to c:/msysgit.  For
test t5560, the path being requested from git-http-backend is specified
via environment variable PATH_INFO as a URL path of the form
/repo.git/foobar, which git-http-backend combines with GIT_PROJECT_ROOT
to determine the actual physical path within the repository. This is a
case where MSYS's conversion of the path-like value of PATH_INFO causes
harm, for two reasons.  First, the resulting converted path, when joined
with GIT_PROJECT_ROOT is bogus (for instance,
"C:/msysgit/git/t/trash-zzz/C:/msysgit/repo.git/HEAD").  Second, the
converted PATH_INFO path is rejected by git-http-backend as an 'alias'
due to validation failure on the part of daemon_avoid_alias().
Unfortunately, the standard work-around of doubling the leading slash
(i.e. //repo.git/foobar) to suppress MSYS path conversion works only for
command-line arguments, but not for environment variables.
Consequently, side step the problem by instead passing git-http-backend
an already-constructed full path rather than components GIT_PROJECT_ROOT
and PATH_INFO.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>