odb: Do absolute-path resolution via rev-parse

Instead of manually resolving from `self.workdir /`.

According to
mystor#82 (comment),
the current minimum supported git version is 2.5 or 2.7.3.

This means we can use --absolute-git-dir, which was introduced in 2.13.

--path-format=absolute was introduced in git 2.31.0, which at the time
of this writing is very recent, so a comment has been added to update
that remaining case when it's appropriate.

gitrevise/odb.py

@@ -162,7 +162,7 @@ def __init__(self, cwd: Optional[Path] = None) -> None:
         self._tempdir = None
 
         self.workdir = Path(self.git("rev-parse", "--show-toplevel", cwd=cwd).decode())
-        self.gitdir = self.workdir / Path(self.git("rev-parse", "--git-dir").decode())
+        self.gitdir = Path(self.git("rev-parse", "--absolute-git-dir").decode())
 
         # XXX(nika): Does it make more sense to cache these or call every time?
         # Cache for length of time & invalidate?
@@ -274,6 +274,8 @@ def get_tempdir(self) -> Path:
 
     def git_path(self, path: "StrOrBytesPath") -> Path:
         """Get the path to a file in the .git directory, respecting the environment"""
+        # NOTE: For git >= 2.31.0, this manual reconstruction from self.workdir
+        # can be replaced with "git rev-parse --path-format=absolute --git-path …"
         return self.workdir / self.git("rev-parse", "--git-path", path).decode()
 
     def new_commit(