Fix incorrect review comment diffs

[jpraet]

Fixes #13683.

The diff snippet that provides context for a code review comment on the pull request timeline page used to be calculated based on the headCommitID. But in 1.13, with PR #13448, this changed to the commitID from the blame for the commented line, which seems to cause these incorrect review comment diff snippets.

This PR reverts back to using the headCommitID for obtaining the diff.

[codecov-io]

Codecov Report

Merging #14002 (16301f2) into master (88b585c) will decrease coverage by 0.02%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master   #14002      +/-   ##
==========================================
- Coverage   42.22%   42.20%   -0.03%     
==========================================
  Files         710      710              
  Lines       77258    77259       +1     
==========================================
- Hits        32626    32610      -16     
- Misses      39268    39271       +3     
- Partials     5364     5378      +14     

Impacted Files	Coverage Δ
services/pull/review.go	51.07% <33.33%> (+0.35%)	⬆️
modules/util/timer.go	42.85% <0.00%> (-42.86%)	⬇️
modules/queue/unique_queue_channel.go	58.06% <0.00%> (-6.46%)	⬇️
modules/notification/ui/ui.go	93.05% <0.00%> (-2.78%)	⬇️
services/pull/check.go	49.63% <0.00%> (-1.46%)	⬇️
models/notification.go	66.25% <0.00%> (-0.99%)	⬇️
models/error.go	38.49% <0.00%> (-0.82%)	⬇️
models/issue_comment.go	52.26% <0.00%> (-0.46%)	⬇️
modules/notification/mail/mail.go	39.08% <0.00%> (ø)
services/mailer/mail.go	61.29% <0.00%> (+1.07%)	⬆️
... and 1 more

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