feat(gitter): affected commits graph walking (google#4974)

Add graph related logic and find affected commits to gitter

Definition of affected commits:
> A `commit` is affected when: from at least one `introduced` that is an
ancestor of the `commit`, there is no path between them that passes
through a fix. A fix can either be a `fixed` commit, or the children of
a `lastAffected` commit.

### Key changes
New endpoint: 
- `POST /affected-commits`: 
- Accepts a list of events (each containing an eventType (introduced,
fixed, lastAffected, or limit) and commit hash)
  - Returns a list of affected commits and refs (tags and branches)

Cherry-pick Detection (optional): 
Expand introduced, fixed, limit commits to include their cherry-picked
equivalents (determined with PatchID)

Author: Ly-Joey

deployment/clouddeploy/gke-workers/base/gitter.yaml

@@ -29,6 +29,8 @@ spec:
           - "--port=8888"
           - "--work-dir=/work/gitter"
           - "--fetch-timeout=1h"
+          - "--repo-cache-ttl=1h"
+          - "--repo-cache-max-cost=100GiB"
         env:
         - name: GOMEMLIMIT
           value: "100GiB"

deployment/clouddeploy/gke-workers/environments/oss-vdb-test/gitter.yaml

@@ -11,6 +11,8 @@ spec:
           - "--port=8888"
           - "--work-dir=/work/gitter"
           - "--fetch-timeout=1h"
+          - "--repo-cache-ttl=1h"
+          - "--repo-cache-max-cost=100GiB"
         env:
           - name: GOOGLE_CLOUD_PROJECT
             value: oss-vdb-test

deployment/clouddeploy/gke-workers/environments/oss-vdb/gitter.yaml

@@ -11,6 +11,8 @@ spec:
           - "--port=8888"
           - "--work-dir=/work/gitter"
           - "--fetch-timeout=1h"
+          - "--repo-cache-ttl=1h"
+          - "--repo-cache-max-cost=100GiB"
         env:
           - name: GOOGLE_CLOUD_PROJECT
             value: oss-vdb