logging(gitstart): migrate gitserver logging

[bobheadxi]

Problem statement

We are undergoing a migration to our new logging library, github.com/sourcegraph/sourcegraph/lib/log. This library has a number of significant changes that requires manual migration:

1. Strongly typed API for adding contextual fields (using log.String, etc.)
2. No global loggers are allowed (loggers are created with log.Scoped and then passed around to handle logging concerns)

For more details, see https://docs.sourcegraph.com/dev/how-to/add_logging .

The main existing logger usage to replace are calls to log15..

Success criteria

All ~125 occurrences of log15 calls in files that appear related to gitserver are replaced with logging calls to an appropriately scoped log.Logger instance, e.g. ones held by the parent struct:

https://sourcegraph.com/search?q=context:global+r:%5Egithub%5C.com/sourcegraph/sourcegraph%24++content:%22log15.%22+lang:go+f:gitserver&patternType=literal

For an example migration, see #35516 and #35759

Green CI builds.

Time estimate

16 hours (~2 days)

[gitstart-app]

Here is the GitStart Ticket for this issue: https://app.gitstart.com/clients/sourcegraph/tickets/SG-35514