setup: improve error diagnosis for invalid .git files

'read_gitfile_gently()' treats any non-regular file as
'READ_GITFILE_ERR_NOT_A_FILE' and fails to discern between 'ENOENT'
and other stat failures. This flawed error reporting is noted by two
'NEEDSWORK' comments.

Address these comments by introducing two new error codes:
'READ_GITFILE_ERR_MISSING'(which groups the "file missing" scenarios
together) and 'READ_GITFILE_ERR_IS_A_DIR':

1. Update 'read_gitfile_error_die()' to treat 'IS_A_DIR', 'MISSING',
'NOT_A_FILE' and 'STAT_FAILED' as non-fatal no-ops. This accommodates
intentional non-repo scenarios (e.g., GIT_DIR=/dev/null).

2. Explicitly catch 'NOT_A_FILE' and 'STAT_FAILED' during
discovery and call 'die()' if 'die_on_error' is set.

3. Unconditionally pass '&error_code' to 'read_gitfile_gently()'.

4. Only invoke 'is_git_directory()' when we explicitly receive
   'READ_GITFILE_ERR_IS_A_DIR', avoiding redundant checks.

Additionally, audit external callers of 'read_gitfile_gently()' in
'submodule.c' and 'worktree.c' to accommodate the refined error codes.

Signed-off-by: Tian Yuchen <a3205153416@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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