Make capitalization check work with 'nocasematch' on

Bash has an option, 'nocasematch' (see 'help shopt') that makes pattern
matching case-insensitive. So, the check for "subject line capitalized"
  [[ ${subject} =~ ^[[:lower:]] ]]
will report a non-capitalized subject whenever the subject line starts
with an ASCII letter, lowercase or uppercase, that is, basically always.

As far as I know, 'nocasematch' is rarely on. However, if it is on,
without this fix it would produce a very confusing error message.
For example,

$ git commit -m "Add HTTP client"
ERROR: commit message not properly formatted
- line 1: subject not capitalized
  Add HTTP client

lint-commit-msg

@@ -499,6 +499,7 @@ while true; do
         # errors that only confuse the user.
 
         # Capitalization check
+        shopt -u nocasematch
         [[ ${subject} =~ ^[[:lower:]] ]] &&
             append_error \
                 "${error_subject_not_capitalized}" \

test/cases/subject-line-capitalization.test

@@ -72,3 +72,14 @@ EOF
     call lint-commit-msg msg.txt
     expect_success
 }
+
+test_capitalized_with_nocasematch() {
+    cat > msg.txt <<EOF
+Start with an uppercase letter
+EOF
+    shopt -s nocasematch
+    export BASHOPTS
+
+    call lint-commit-msg msg.txt
+    expect_success
+}