gitserver: grpc: add new ChangedFiles method to gitserver client

[ggilmore]

Part of #60654

This PR builds on #62216, and creates a new ChangedFiles method in the gitserver client that calls the new gRPC method that was introduced in #62216.

Note: This PR doesn't change / remove any existing callers of DiffSymbols. I made those changes in future stacked PRs to ensure that they're easy to review.

Test plan

New Unit tests

[ggilmore]

• searcher: remove unused diff.ParseGitDiffNameStatus package #62363
• gitserver: grpc: remove now unused DiffSymbols func #62360
• worker: embeddings: replace usage of old gitserver.DiffSymbols client method with new gitserver.ChangedFiles method #62359
• searcher: replace usage of gitserver.DiffSymbols with new gitserver.ChangedFiles client method #62358
• symbols: replace usage of gitserver client's DiffSymbols method with new ChangedFiles method #62355
• gitserver: grpc: add new ChangedFiles method to gitserver client #62354 👈
• gitserver: grpc: create changed files RPC implementation for gRPC server #62262
• gitserver: grpc: port DiffSymbols from client as new GitBackend.ChangedFiles() func #62252
• gitserver: grpc: swap client implementation of GetAheadBehind from Exec to bespoke gRPC endpoint #62217
• gitserver: grpc: create server implementation for GetBehindAhead #62216
• gitserver: grpc: port GetBehindAhead from client to gitcli backend #62212
• codeinsights: remove now useless empty repo string error check #62302
• gitserver: grpc: switch clientImplementor to use new dedicated gRPC method for FirstEverCommit #62173
• gitserver: grpc: create FirstEverCommit RPC method and server implementation #62169
• gitserver: port FirstEverCommit functionality from client to git cli backend #62165
• gitserver: grpc: create exhaustive logger for repository service #62158
• main

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[ggilmore]

@sourcegraph/source Can you please take another look at this PR (and all the upstack PRs that use it)? I refactored the approach here to use iterators.

[ggilmore]

Merge activity

• May 7, 1:45 PM EDT: Graphite rebased this pull request after merging its parent, because this pull request is set to merge when ready.
• May 7, 1:58 PM EDT: @ggilmore merged this pull request with Graphite.