Windows: Treat Windows style path names.

GIT's guts work with a forward slash as a path separators. We do not change
that. Rather we make sure that only "normalized" paths enter the depths
of the machinery.

We have to translate backslashes to forward slashes in the prefix and in
command line arguments. Fortunately, all of them are passed through
functions in setup.c.

A macro has_dos_drive_path() is defined that checks whether a path begins
with a drive letter+colon combination. This predicate is always false on
Unix. Another macro is_dir_sep() abstracts that a backslash is also a
directory separator on Windows.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>

cache.h

@@ -521,7 +521,7 @@ int safe_create_leading_directories(char *path);
 char *enter_repo(char *path, int strict);
 static inline int is_absolute_path(const char *path)
 {
-	return path[0] == '/';
+	return path[0] == '/' || has_dos_drive_prefix(path);
 }
 const char *make_absolute_path(const char *path);
 const char *make_nonrelative_path(const char *path);