cleanup_path: force forward slashes on Windows

Git prefers forward slashes as directory separators across all
platforms. On Windows, the backslash is the native directory separator,
but all Windows versions supported by Git also accept the forward slash
in all but rare circumstances. Our tests expect forward slashes. Git
generates relative paths with forward slashes. Forward slashes are more
convenient to use in shell scripts.

For these reasons, we enforced forward slashes in `interpolate_path()`
in 5ca6b7b (config --show-origin: report paths with forward slashes,
2016-03-23). However, other code paths may generate paths containing
backslashes. For example, `config --show-origin` prints the XDG config
path with mixed slashes on Windows:

$ git config --list --show-origin
file:C:/Program Files/Git/etc/gitconfig         system.foo=bar
file:"C:\\Users\\delilah/.config/git/config"    xdg.foo=bar
file:C:/Users/delilah/.gitconfig                home.foo=bar
file:.git/config                                local.foo=bar

Let's enforce forward slashes in all code paths that directly or
indirectly call `cleanup_path()` by modifying it to use
`convert_slashes()` on Windows. Since `convert_slashes()` modifies the
path in-place, change the argument and return type of `cleanup_path()`
from `const char *` to `char *`. All existing callers of
`cleanup_path()` pass `char *` anyways, so this change is compatible.

The next patch, config: test home and xdg files in `list --global`, will
assert that the XDG config path uses forward slashes.

Suggested-by: Johannes Schindelin <[email protected]>
Signed-off-by: Delilah Ashley Wu <[email protected]>
Reviewed-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: delilahw

path.c

@@ -40,13 +40,17 @@ static struct strbuf *get_pathname(void)
 	return sb;
 }
 
-static const char *cleanup_path(const char *path)
+static char *cleanup_path(char *path)
 {
 	/* Clean it up */
-	if (skip_prefix(path, "./", &path)) {
+	if (skip_prefix(path, "./", (const char **)&path))
 		while (*path == '/')
 			path++;
-	}
+
+#ifdef GIT_WINDOWS_NATIVE
+	convert_slashes(path);
+#endif
+
 	return path;
 }