Use releaseBranch variable instead of the hard-coded "main" in git log command

[mmizutani]

This PR replaces a hard-coded main branch name, "main", with the releaseBranch variable in a git command in tagpr.go.

With the original implementation, when a release branch name different from "main" (e.g., "master" or "develop") is specified in the .tagpr config file, we encounter the following error during the execution of the Songmu/tagpr@v1 action:

...
Switched to a new branch 'tagpr-from-v1.24.0'
git [add -f .tagpr]
git [commit --allow-empty -am [tagpr] prepare for the next release]
[tagpr-from-v1.24.0 b505e8d25] [tagpr] prepare for the next release

git [log --no-merges --pretty=format:%h %s main..origin/tagpr-from-v1.24.0]
fatal: ambiguous argument 'main..origin/tagpr-from-v1.24.0': unknown revision or path not in the working tree.  👈
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

Such a git log ... command error is currently ignored and results in an empty diff ("What's Changed" section) in the automated pull request as follows:

This PR corrects the git log command so that tagpr successfully retrieves non-merge commits even when the name of the release branch is anything other than "main".

[Songmu]

oops...
Thank you!

[mmizutani]

@Songmu
Thank you for the new version release. 😃
Nice tool!