issue #7520 : fix MissingObjectException when switching git projects

[mattcasters]

What

Fixes org.eclipse.jgit.errors.MissingObjectException in the Git Perspective when switching Hop projects (or explorer roots) that map to different local git repositories.

Root cause

The Git Perspective is a long-lived singleton. On project switch it opened the new repository and then called updateGui() while the history table still held RevCommit selection from the previous repo. isCommitInCurrentBranch re-parsed those ids against the newly opened object database → Missing unknown <other-project-SHA>.

This is a 2.18 Git Perspective lifecycle bug, not a general JGit inability to read commit-graph / multi-pack-index / bitmaps. See the analysis on #7520.

Changes

• A: Clear ref/history/file UI state first on refresh; rebuild for the current repo; call updateGui() last.
• B: Close the previous UIGit repository on rootChanged (null-safe closeRepo()).
• C: Harden branch membership check: objectDatabase.has(id) / quiet MissingObjectException for foreign ids.
• Small headless JGit unit test for the branch-check helper (two temp repos; no native git, no SWT).

How was this tested?

• ./mvnw -pl plugins/misc/git -am test -Dtest=GitPerspectiveBranchCheckTest -Dsurefire.failIfNoSpecifiedTests=false
• Compile of the git plugin module

Related

Fixes #7520

[hansva]

Going to keep this open until we have a reply in #7520, as I do not have the correct setup to properly test this fix.

[mattcasters]

Set to draft until we have confirmation.

[hansva]

Merging as he can't test on code only, this way he will have a snapshot build