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 using 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 backslashes 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 call
`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.

Suggested-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Delilah Ashley Wu <delilahwu@microsoft.com>

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;
 }