Fix Asciidoctor footnoteref deprecation

Asciidoctor has preferred another way to write this style of footnotes
for years [1]. The general pattern of the change is

    -footnoteref:[myid,my text].
    +footnote:myid[my text].

However, unlike the footnoteref macro prints "my text" verbatim whereas
the footnote macro processes it like extra text, so the URL used here
has to be escaped to disable autolinking.

[1] asciidoctor/asciidoctor#3114

doc/commitmsgfmt.1.adoc

@@ -3,7 +3,7 @@
 :man source: commitmsgfmt {version}
 :man manual: User Commands
 :self: pass:q[*commitmsgfmt*]
-:uri-algo: https://en.wikipedia.org/w/index.php?title=Line_wrap_and_word_wrap&oldid=838974524#Minimum_number_of_lines
+:uri-algo: \https://en.wikipedia.org/w/index.php?title=Line_wrap_and_word_wrap&oldid=838974524#Minimum_number_of_lines
 
 == Name
 
@@ -76,8 +76,8 @@ single word, or for instance a long URL--will not be wrapped. Indentation
 spaces are ignored in this context.
 
 {self} currently uses the traditional greedy _minimum number of lines_ wrapping
-algorithm{empty}footnoteref:[wrap-algo,{uri-algo}], like *fmt*(1) and Vim, for
-ease of implementation and performance comparability. This may change.
+algorithm{empty}footnote:wrap-algo[{uri-algo}], like *fmt*(1) and Vim, for ease
+of implementation and performance comparability. This may change.
 
 === Subject line