7125: Support merging master/main into other branches in GitGraph

[darshanr0107]

📑 Summary

This PR fixes an issue where GitGraph diagrams did not correctly handle merges from master/main into other branches, preventing proper merge history visualization.
Resolves #7125

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Latest commit: 256e81b

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commit: 256e81b

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 3.55%. Comparing base (85a13da) to head (256e81b).
⚠️ Report is 61 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #7155   +/-   ##
=======================================
  Coverage     3.55%   3.55%           
=======================================
  Files          473     474    +1     
  Lines        47496   47496           
  Branches       730     731    +1     
=======================================
  Hits          1687    1687           
  Misses       45809   45809           

Flag	Coverage Δ
unit	3.55% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
packages/mermaid/src/diagrams/git/gitGraphAst.ts	0.00% <ø> (ø)

... and 1 file with indirect coverage changes

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Build	Status	Details	Updated (UTC)
default (Inspect)	⚠️ Changes detected (Review)	1 added	Nov 12, 2025, 6:45 AM