Fix relative path compatibility for --replace-text and bfg_args.repo

Users could specify relative paths on the command line, and then also
provide a directory other than '.' for the repo.  Since we did an
unconditional os.chdir() to move into the repo, that would invalidate
the original relative paths.  Fix that by changing the relative paths
into absolute paths.

Signed-off-by: 林博仁(Buo-ren Lin) <Buo.Ren.Lin@gmail.com>
[en: tweaked commit message to explain the problem]
Signed-off-by: Elijah Newren <newren@gmail.com>

contrib/filter-repo-demos/bfg-ish

@@ -371,6 +371,7 @@ class BFG_ish:
     bfg_args = self.parse_options()
     preserve_refs = self.get_preservation_info(bfg_args.preserve_ref_tips)
 
+    work_dir = os.getcwd()
     os.chdir(bfg_args.repo)
     bfg_args.delete_files = java_to_fnmatch_glob(bfg_args.delete_files)
     bfg_args.delete_folders = java_to_fnmatch_glob(bfg_args.delete_folders)
@@ -395,6 +396,9 @@ class BFG_ish:
       extra_args += ['--preserve-commit-hashes']
     new_replace_file = None
     if bfg_args.replace_text:
+      if not os.path.isabs(bfg_args.replace_text):
+        bfg_args.replace_text = os.path.join(work_dir, bfg_args.replace_text)
+
       new_replace_file = self.convert_replace_text(bfg_args.replace_text)
       rules = fr.FilteringOptions.get_replace_text(new_replace_file)
       self.replacement_rules = rules
@@ -432,6 +436,9 @@ class BFG_ish:
       if not fr.GitUtils.is_repository_bare('.'):
         need_another_reset = True
 
+    if not os.path.isabs(os.fsdecode(bfg_args.repo)):
+      bfg_args.repo = os.fsencode(os.path.join(work_dir, os.fsdecode(bfg_args.repo)))
+
     fr.RepoFilter.cleanup(bfg_args.repo, repack=True, reset=need_another_reset)
 
 if __name__ == '__main__':