file-manager: fix double-free and relative path crash for submodule gitdir pointers

[Inspirati]

Problem

When Caja enumerates a directory whose subdirectories contain a .git file
(rather than a .git directory — as used by git submodules and linked
worktrees), get_git_branch() crashes intermittently via heap corruption.

The root cause is a double-free:

1. Inside get_git_branch(), when .git is a regular file the code calls
   g_free(git_path) on its own const char * parameter (to free the
   caller's allocation before reassigning the local variable).
2. The caller then calls g_free(git_path) again on the same pointer after
   get_git_branch() returns.

The resulting heap corruption is non-deterministic — it manifests as a crash
elsewhere in the process, not at the site of the double-free, which is why a
desktop code review found no obvious NULL-dereference path in the feature code.

A secondary issue: g_build_filename() does not canonicalise ../ components
in relative gitdir: paths. If the resolved path does not exist on disk
(e.g. an uninitialised submodule), realpath() returns NULL; without a guard
this would be passed into subsequent GLib calls.

Fix

• Introduce a resolved_gitdir local variable to hold the heap-allocated
  resolved path. The git_path parameter (which belongs to the caller) is
  never freed inside the function.
• Use realpath() to canonicalise relative gitdir: paths containing ../
  components. The result is copied into GLib-managed memory with g_strdup()
  and the malloc()-allocated realpath() buffer is freed with free().
• Guard against a NULL return from realpath() — when the target path is
  absent on disk, effective_gitdir is set to NULL and the subsequent
  g_file_test() call is skipped cleanly.
• Add #include <stdlib.h> for realpath() and free().

Crash reproduction

Navigate to a parent directory in Caja icon view that contains a subdirectory
with a .git file whose gitdir: line holds a relative path
(e.g. gitdir: ../../../../../.git/modules/...). With the Display Git Branch
preference enabled, Caja crashes on directory enumeration. After this fix,
enumeration completes without crashing.

[lukefromdc]

Looking closely at this due to its AI origin. I don't see any obvious AI screwups here, but I don't know this part of the code well enough to follow the flow of it from just what we see here

[vkareh]

The commit in this PR is included in #1893, which I reviewed earlier today. I think we should leave this here open for now until we decide whether we use one or the other (the fix is valid in either case, with some small changes)