read-cache: pass 'repo' to 'ce_mode_from_stat()'

read-cache: pass 'repo' to 'ce_mode_from_stat()'

The ce_mode_from_stat() function is a performance-critical static
inline helper in 'read-cache.h'. As we migrate configuration
variables into the repository struct, this helper needs access
to the repository context.

Update the signature of ce_mode_from_stat() to take a 'struct
repository *' parameter, and update all callers to pass the
appropriate repository instance.

To prepare for the overhead of replacing cheap global variable
accesses with getter functions, the boolean expressions are
reordered to evaluate 'S_ISREG(mode)' first.

While at it, add a comment for ce_mode_from_stat().

Mentored-by: Christian Couder <redacted>
Mentored-by: Ayush Chandekar <redacted>
Mentored-by: Olamide Caleb Bello <redacted>
Signed-off-by: Tian Yuchen <redacted>
Signed-off-by: Junio C Hamano <redacted>