Meta: Ignore title length of revert commits

Even if a commit message title previously fit into the 72 character
limit it may go over that with 'Revert ""' added - asking the author to
tweak the commit message is not appropriate in that case.

Since additional context can be added to the commit message body the
usual length restriction applies to the remaining lines.

Also restructure the code to group checks based on the line number we're
currently looking at.

Meta/lint-commit.sh

@@ -30,36 +30,43 @@ while read -r line; do
   ((line_number += 1))
   line_length=${#line}
 
-  if [[ $line_number -eq 2 ]] && [[ $line_length -ne 0 ]]; then
-    error "Empty line between commit title and body is missing"
-  fi
+  if [[ $line_number -eq 1 ]]; then
+    merge_commit_pattern="^Merge branch"
+    if (echo "$line" | grep -E -q "$merge_commit_pattern"); then
+      error "Commit is a git merge commit, use the rebase command instead"
+    fi
 
-  merge_commit_pattern="^Merge branch"
-  if [[ $line_number -eq 1 ]] && (echo "$line" | grep -E -q "$merge_commit_pattern"); then
-    error "Commit is a git merge commit, use the rebase command instead"
-  fi
+    category_pattern='^(Revert "|\S+: )'
+    if (echo "$line" | grep -E -v -q "$category_pattern"); then
+      error "Missing category in commit title (if this is a fix up of a previous commit, it should be squashed)"
+    fi
 
-  category_pattern='^(Revert "|\S+: )'
-  if [[ $line_number -eq 1 ]] && (echo "$line" | grep -E -v -q "$category_pattern"); then
-    error "Missing category in commit title (if this is a fix up of a previous commit, it should be squashed)"
-  fi
+    revert_pattern='^Revert "'
+    if [[ $line_length -gt 72 ]] && (echo "$line" | grep -E -v -q "$revert_pattern"); then
+      error "Commit title is too long (maximum allowed is 72 characters)"
+    fi
 
-  title_case_pattern="^\S.*?: [A-Z0-9]"
-  if [[ $line_number -eq 1 ]] && (echo "$line" | grep -E -v -q "$title_case_pattern"); then
-    error "First word of commit after the subsystem is not capitalized"
-  fi
+    title_case_pattern="^\S.*?: [A-Z0-9]"
+    if (echo "$line" | grep -E -v -q "$title_case_pattern"); then
+      error "First word of commit after the subsystem is not capitalized"
+    fi
 
-  if [[ $line_number -eq 1 ]] && [[ "$line" =~ \.$ ]]; then
-    error "Commit title ends in a period"
-  fi
-
-  url_pattern="([a-z]+:\/\/)?(([a-zA-Z0-9_]|-)+\.)+[a-z]{2,}(:\d+)?([a-zA-Z_0-9@:%\+.~\?&\/=]|-)+"
-  if [[ $line_length -gt 72 ]] && (echo "$line" | grep -E -v -q "$url_pattern"); then
-    error "Commit message lines are too long (maximum allowed is 72 characters)"
-  fi
+    if [[ "$line" =~ \.$ ]]; then
+      error "Commit title ends in a period"
+    fi
+  elif [[ $line_number -eq 2 ]]; then
+    if [[ $line_length -ne 0 ]]; then
+      error "Empty line between commit title and body is missing"
+    fi
+  else
+    url_pattern="([a-z]+:\/\/)?(([a-zA-Z0-9_]|-)+\.)+[a-z]{2,}(:\d+)?([a-zA-Z_0-9@:%\+.~\?&\/=]|-)+"
+    if [[ $line_length -gt 72 ]] && (echo "$line" | grep -E -v -q "$url_pattern"); then
+      error "Commit message lines are too long (maximum allowed is 72 characters)"
+    fi
 
-  if [[ "$line" == "Signed-off-by: "* ]]; then
-    error "Commit body contains a Signed-off-by tag"
+    if [[ "$line" == "Signed-off-by: "* ]]; then
+      error "Commit body contains a Signed-off-by tag"
+    fi
   fi
 
 done <"$commit_file"