fix: add stall detection and faster retries to git fetch

[devin-ai-integration]

Summary

Addresses checkout step timeouts (e.g. bnk-dev/bank run 27376021190 where 3 jobs failed and most "successful" checkouts took 100-178s). The root cause is a stalled git fetch consuming the entire step-level timeout with no chance for the retry mechanism to help.

Two changes to GitCommandManager.fetch():

1. Stall detection via git's built-in HTTP low-speed abort:

   -c http.lowSpeedLimit=1000 -c http.lowSpeedTime=15
   

   If transfer speed drops below 1 KB/s for 15 consecutive seconds, git aborts the fetch attempt. Without this, a hung connection blocks until the step timeout (typically 3 minutes) kills everything — only one attempt ever runs.

2. Faster retry cadence — RetryHelper(3, 1, 5) instead of the default (3, 10, 20). With a 3-minute step timeout, the old 10-20s inter-attempt sleeps wasted 20-40s. New 1-5s sleeps give all 3 attempts a fair chance within the budget:

   • Old worst case: attempt₁ stalls 15s + sleep 20s + attempt₂ stalls 15s + sleep 20s + attempt₃ 15s = 85s
   • New worst case: attempt₁ stalls 15s + sleep 5s + attempt₂ stalls 15s + sleep 5s + attempt₃ 15s = 55s

Data context (ClickHouse, last 6h for bnk-dev in us-west): 7187 successful checkouts avg 41s, 31 failures all at ~189s (= step timeout). Multiple customers affected across us-west, not region-specific.

Link to Devin session: https://app.devin.ai/sessions/88dbc85a54eb4f22b1bcf057a8509fcf

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[devin-ai-integration]

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