Migrate commit_count to always use revisions parameter internally

Summary

This MR implements the plan outlined in #587518 (closed) to migrate the commit_count method to internally convert legacy revision and all parameters to the new unified revisions parameter when calling Gitaly.

Changes

Feature Flag

Added count_commits_use_revisions_only feature flag (disabled by default) to control the rollout.

Updated commit_count Method

When the feature flag is enabled, the method now:

• Converts ref parameter → revisions: [ref]
• Converts all: true → revisions: ['--all']
• Passes revisions parameter through unchanged

Backward Compatibility

• External API remains unchanged - all existing calls continue to work
• When flag is disabled, uses the existing behavior
• When flag is enabled, converts parameters internally before calling Gitaly

Test Coverage

Added comprehensive tests for the new conversion logic:

• Single revision conversion
• all: true conversion to --all
• revisions parameter passthrough
• Error handling for missing revisions
• Compatibility with filters (before, after, path, max_count, first_parent)
• UTF-8 encoding preservation

Migration Examples

External API stays the same:

# These all continue to work exactly as before
commit_count('master')           # Internally: revisions: ['master']
commit_count(nil, all: true)     # Internally: revisions: ['--all']
commit_count(nil, revisions: ['feature-a', 'feature-b'])  # Internally: revisions: ['feature-a', 'feature-b']

Benefits

• Simplified Gitaly code: Single code path in Gitaly for handling revisions
• Better maintainability: One way to handle all revision inputs
• Backward compatible: No changes to external API
• Future-proof: All code already using the modern parameter internally

Related

• Issue: #587518 (closed)
• Parent issue: #586893 (closed)
• Original MR: !219416 (merged)