[Chapel Merge] Update 'updateGITLOG' to use 'main' rather than 'm

Branch: refs/heads/main
Revision: 05417f3
Author: bradcray
Log Message:

Merge pull request #18364 from bradcray/updategitlog-main

Update 'updateGITLOG' to use 'main' rather than 'master' if it exists

[trivial, not reviewed / orthogonal to the release / potentially not used by anyone but me]

This updates my utility script that lists all merge commits in a
directory using Brad-approved format so that it uses the branch
'main' if it exists rather than 'master', as it traditionally has. I ran
into this while prepping instructions on contributing entries to
CHANGES.md and the release notes outline.

Modified Files:
M util/devel/updateGITLOG

Compare: https://github.com/chapel-lang/chapel/compare/12b1382cafce...05417f37c2a4