feat: go template function uniqueOlderCommits/uniqueNewerCommits is available - #211

• They allow to remove duplicated log entries.

Closes #29

What does this do / why do we need it?

• Introduce new go template custom functions uniqueOlderCommits and uniqueNewerCommits. They removes duplicated commits / log entries. "Duplicated" is evaluated with fields specified. For example, if your changelog entries consists of .Scope and .Subject, (uniqueOlderCommits .Commits "Scope" "Subject") is expected.
• There are cases log entries are duplicated. See Expect to filter duplicate commits! #29. We had to write really complicated go templates to remove those duplication. These new functions let us do that easily.

We can rewrite .chglog/CHANGELOG.tpl.md in git-chglog like this:

diff --git a/.chglog/CHANGELOG.tpl.md b/.chglog/CHANGELOG.tpl.md
index 5683d03..db302fe 100755
--- a/.chglog/CHANGELOG.tpl.md
+++ b/.chglog/CHANGELOG.tpl.md
@@ -5,7 +5,7 @@
 {{ if .Unreleased.CommitGroups -}}
 {{ range .Unreleased.CommitGroups -}}
 ### {{ .Title }}
-{{ range .Commits -}}
+{{ range (uniqueOlderCommits .Commits "Scope" "Subject") -}}
 - {{ if .Scope }}**{{ .Scope }}:** {{ end }}{{ .Subject }}
 {{ end }}
 {{ end -}}
@@ -17,7 +17,7 @@
 ## {{ if .Tag.Previous }}[{{ .Tag.Name }}]{{ else }}{{ .Tag.Name }}{{ end }} - {{ datetime "2006-01-02" .Tag.Date }}
 {{ range .CommitGroups -}}
 ### {{ .Title }}
-{{ range .Commits -}}
+{{ range (uniqueOlderCommits .Commits "Scope" "Subject") -}}
 - {{ if .Scope }}**{{ .Scope }}:** {{ end }}{{ .Subject }}
 {{ end }}
 {{ end -}}

How this PR fixes the problem?

• The underlying problem of Expect to filter duplicate commits! #29 is:
  • We have to filter Commit to remove duplication.
  • But it's really difficult to handle structured data with go template: Sprig doesn't work for structured data. Go template doesn't provide mechanisms like lambda functions.
  • What we want to do is: Detect duplication only with specific fields in Commit. For example, "Scope" and "Subject". Or it may be JiraIssueID (for the case only one entry for one ticket).
• I believe uniqueOlderCommits and uniqueNewerCommits resolves that.

What should your reviewer look out for in this PR?

• Let me know if better function names.
• Please propose better descriptions in README.md. This feature is too complicated for my English.

Check lists

• Test passed
• Coding style (indentation, etc)

Additional Comments (if any)

• Older and Newer means the commit order. I believe we also need fix: the sort order of commits is unstable #210 to have this feature fully functioned.
• This code isn't good in performance (O(n^2)). But this doesn't need so much performance and I believe it's better to keep easy to read and easy to maintenance.

Which issue(s) does this PR fix?

fixes #29