fix(post-upgrade): preserve executable file modes

[zharinov]

Changes

postUpgradeTasks.fileFilters collected changed file contents without their executable state. As a result, a generated executable file or chmod +x change could still be committed as 100644.

When Git's per-repository core.fileMode setting says filesystem modes are reliable, preserve the owner's execute bit by recording isExecutable: true. Otherwise leave the mode unspecified so the existing Git index state remains unchanged. This fix is intentionally additive and does not implement executable removal.

Context

Please select one of the following:

• This closes an existing Issue, Closes: postUpgradeTasks fileFilters drop the executable bit #44501
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

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