dev/sg: only look for revisions in main

Created by: bobheadxi

Fix update checks and sg version changelog erroneously detecting changes in branches as new sg changes.

Test plan

Tested the raw git log commands.

Before, git log contains unexpected revision on a branch I'm working on:

~/Projects/sourcegraph/sourcegraph sg/fix-version-check
❯ git log --pretty=short --no-merges 1e32aa0711560187d8431f2d4510edc161c0d91a.. -- ./dev/sg           
commit 1a40346d264bfc57b9f1dac5cf724f03bd64501b (HEAD -> sg/fix-version-check, origin/sg/fix-version-check)
Author: Robert Lin <[email protected]>

    dev/sg: only look for revisions in main

commit 5c166e94e349aa3297ef95dcab487527288112bb
Author: Thorsten Ball <[email protected]>

    sg migration: prefer ENV vars from config, fallback to process env (#31926)

After:

~/Projects/sourcegraph/sourcegraph sg/fix-version-check
❯ git log --pretty=short --no-merges 1e32aa0711560187d8431f2d4510edc161c0d91a..origin/main -- ./dev/sg
commit 5c166e94e349aa3297ef95dcab487527288112bb
Author: Thorsten Ball <[email protected]>

    sg migration: prefer ENV vars from config, fallback to process env (#31926)

Similarly for rev-list:

~/Projects/sourcegraph/sourcegraph sg/fix-version-check
❯ git rev-list --pretty=short --no-merges 1e32aa0711560187d8431f2d4510edc161c0d91a..origin/main -- ./dev/sg
commit 5c166e94e349aa3297ef95dcab487527288112bb
Author: Thorsten Ball <[email protected]>

    sg migration: prefer ENV vars from config, fallback to process env (#31926)


~/Projects/sourcegraph/sourcegraph sg/fix-version-check
❯ git rev-list --pretty=short --no-merges 1e32aa0711560187d8431f2d4510edc161c0d91a.. -- ./dev/sg 
commit 1a40346d264bfc57b9f1dac5cf724f03bd64501b
Author: Robert Lin <[email protected]>

    dev/sg: only look for revisions in main

commit 5c166e94e349aa3297ef95dcab487527288112bb
Author: Thorsten Ball <[email protected]>

    sg migration: prefer ENV vars from config, fallback to process env (#31926)