[roll] Roll fuchsia [git] Centralize git path resolution and fix regen

Introduce a centralized mechanism to resolve Git paths (like HEAD and
index) in GN, with full support for git worktrees. This replaces
direct accesses to `.git/` files which fail in worktrees because `.git`
is a file pointing to the real git directory.

Also fix the GN auto-regeneration bug by implementing a two-step
resolution in GN:
1. Resolve the paths to the git files (like HEAD and ref) dynamically
   using `resolve_git_paths.py` (which uses `git rev-parse --git-path`
   and is worktree-aware).
2. Pass these resolved paths as `file_dependencies` to the second
   `exec_script` call that resolves the actual revision hashes using
   `resolve_git_revisions.py`.
This forces Ninja to rerun GN when HEAD or the branch ref changes,
without the performance penalty of tracking the `index` file at
GN-time.

Performance (10 runs)
- Before (min: 9.38s, mean: 10.00s)
- After  (min: 9.36s, mean: 10.11s)

Testing:
- Host Unit Tests:
  Run python unit tests for Bazel Git input resolution:
  $ python3 build/bazel/scripts/get_git_head_commit_test.py

- Manual Integration Verification:
  1. GN Auto-Regeneration (Branch):
     Create an empty commit on a branch. The next `fx build` must
     trigger `gn gen` automatically as the branch ref file changed.
  2. GN Auto-Regeneration (Detached HEAD):
     Create an empty commit in detached HEAD state. The next
     `fx build` must trigger `gn gen` automatically as the HEAD
     file changed.
  3. Zircon Fast Rebuild (Index tracking):
     Stage a change (e.g. `git add`). The next `fx build` must
     rebuild the `version-string.txt` target (appending the
     `-dirty` suffix) WITHOUT triggering `gn gen`.

Test: python3 build/bazel/scripts/get_git_head_commit_test.py
Test: Manual integration verification (see description)
Original-Original-Bug: 516876349
TAG=agy
CONV=fbaa9ac0-a008-4115-9c94-a99ca81177f3

Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1646937
Original-Original-Revision: 357056196a7f657e2e6143ad5030e1d6707ba54a
GitOrigin-RevId: c6f8fbb9aff9d4dce42a71af2f2b950422749789
Change-Id: I4c960a78dd29402d071dab4aadb97b3514e365fb