maint: commit-msg: compute UTF-8-aware line-length

maint: commit-msg: compute UTF-8-aware line-length

* scripts/git-hooks/commit-msg: Count UTF-8 characters rather
than bytes to avoid erroneously rejecting as "longer than 72" a
log message line like the UTF-8 one for id.c just prior.  It has
77 bytes but only 67 characters.
(check_msg): Read in "utf8" mode. Also include actual length
in the diagnostic.
(main): Don't loop when stdout is redirected, as it is when
invoked via vc-dwim.
Paul Eggert reported privately both the error of counting bytes
rather than chars and the re_edit loop when failing via vc-dwim.