feat: add new issues.new-from-merge-base option

[ldez]

Adds new issues.new-from-merge-base option.

git merge-base is a better approach than an explicit revision:

gitGraph:
commit id:"a"
commit id:"b" type:HIGHLIGHT
branch my_branch
checkout main
commit id:"c"
checkout my_branch
commit id:"d"
commit  id:"e"tag:"HEAD" type:HIGHLIGHT
checkout main
commit id:"f"
commit id:"g" type:HIGHLIGHT

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When using issues.new-from-rev=main, the diff will be between e and g, which is wrong.

With issues.new-from-merge-base=main, the diff will be between e and b, which is right.

The new option uses git merge-base <ref> HEAD internally. (https://github.com/golangci/revgrep/blob/43995b04fc9795de8791e8a9d82206f95d8acaf1/patch.go#L182)

Reference: https://git-scm.com/docs/git-merge-base

Some real-life examples:

• https://github.com/kubernetes/kubernetes/blob/4bf6cdb60ce764a9ae698c8559da5cf6c3c176dc/hack/verify-golangci-lint.sh#L62
• https://github.com/kubernetes/kubernetes/blob/4bf6cdb60ce764a9ae698c8559da5cf6c3c176dc/hack/verify-golangci-lint.sh#L114
• https://github.com/Kava-Labs/kava/blob/e24bd68ee03ba76c116ba347d5cff777daecc632/build/lint.mk#L16
• https://github.com/hashicorp/packer/blob/6096b86b0e1c9d0c63c6d33cebb2cfb3cf909b83/Makefile#L102
• https://github.com/ionos-cloud/terraform-provider-ionoscloud/blob/6ca4426f97089111b27977ae5be97a6e8ab43da2/GNUmakefile#L15

Fixes #460