Add bin/git-commit-proxy-version (#3071)

Each time we update the proxy from the linkerd2-proxy repo, we make the
change slightly differently. The bin/git-commit-proxy-version does all the
steps needed to update the proxy version up to and including making a
commit to this repo.

The proxy version is now stored in a .proxy-version file and is
consumed directly by Dockerfile-proxy, which both simplifies the
Dockerfile and the update process.

This script formats commit messages and emits output as follows:

```
commit c05198a851f69bdc7007974a0ef1f4c01c98d0ce (HEAD -> ver/proxy-update)
Author: Oliver Gould <ver@buoyant.io>
Date:   Thu Jul 11 17:23:05 2019 +0000

    proxy: Update to linkerd/linkerd2-proxy#3a3ec3b

    * linkerd/linkerd2-proxy#0cc58cd fallback: Clarify fallback layering (linkerd/linkerd2-proxy#288)
    * linkerd/linkerd2-proxy#b71349a Replace `log` and `env-logger` with `tracing` and `tracing-fmt` (linkerd/linkerd2-proxy#277)
    * linkerd/linkerd2-proxy#3a3ec3b Use a constant-time load balancer (linkerd/linkerd2-proxy#266)

diff --git a/.proxy-version b/.proxy-version
index f81f40de..d7faa12d 100644
--- a/.proxy-version
+++ b/.proxy-version
@@ -1 +1 @@
-05b012d
+3a3ec3b
```

.proxy-version

@@ -0,0 +1 @@
+05b012d

Dockerfile-proxy

@@ -4,11 +4,9 @@ FROM gcr.io/linkerd-io/base:2019-02-19.01 as fetch
 RUN apt-get update && apt-get install -y ca-certificates
 WORKDIR /build
 COPY bin/fetch-proxy bin/fetch-proxy
-ARG PROXY_VERSION
-RUN (proxy=$(bin/fetch-proxy $PROXY_VERSION) && \
-    version=$(basename "$proxy" | sed 's/linkerd2-proxy-//') && \
-    mv "$proxy" linkerd2-proxy && \
-    echo "$version" >version.txt)
+COPY .proxy-version proxy-version
+RUN (proxy=$(bin/fetch-proxy $(cat proxy-version)) && \
+    mv "$proxy" linkerd2-proxy)
 
 ## compile proxy-identity agent
 FROM gcr.io/linkerd-io/go-deps:4c8f4294 as golang
@@ -23,7 +21,7 @@ RUN CGO_ENABLED=0 GOOS=linux go install ./proxy-identity
 
 FROM $RUNTIME_IMAGE as runtime
 COPY --from=fetch /build/target/proxy/LICENSE /usr/lib/linkerd/LICENSE
-COPY --from=fetch /build/version.txt /usr/lib/linkerd/linkerd2-proxy-version.txt
+COPY --from=fetch /build/proxy-version /usr/lib/linkerd/linkerd2-proxy-version.txt
 COPY --from=fetch /build/linkerd2-proxy /usr/lib/linkerd/linkerd2-proxy
 COPY --from=golang /go/bin/proxy-identity /usr/lib/linkerd/linkerd2-proxy-identity
 COPY proxy-identity/run-proxy.sh /usr/bin/linkerd2-proxy-run

bin/docker-build-proxy

@@ -13,17 +13,15 @@ rootdir="$( cd $bindir/.. && pwd )"
 . $bindir/_docker.sh
 . $bindir/_tag.sh
 
-dockerfile=$rootdir/Dockerfile-proxy
+dockerfile="$rootdir/Dockerfile-proxy"
 
-validate_go_deps_tag $dockerfile
+validate_go_deps_tag "$dockerfile"
 
 (
     $bindir/docker-build-base
     $bindir/docker-build-go-deps
 ) >/dev/null
 
-# Default to a pinned commit SHA of the proxy.
-PROXY_VERSION="${PROXY_VERSION:-05b012d}"
-
 tag="$(head_root_tag)"
-docker_build proxy $tag $dockerfile --build-arg LINKERD_VERSION=$tag --build-arg PROXY_VERSION=$PROXY_VERSION
+docker_build proxy "$tag" "$dockerfile" \
+  --build-arg "LINKERD_VERSION=$tag"

bin/git-commit-proxy-version

@@ -0,0 +1,49 @@
+#!/bin/bash
+
+set -eu
+
+if [ $# -ne 1 ]; then
+    echo "usage: $(basename $0) PROXY-REVISION" >&2
+    exit 64
+fi
+new_proxy_revision="$1"
+
+bindir="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+rootdir="$( cd $bindir/.. && pwd )"
+
+old_proxy_version="$(tr -d '\n' <"$rootdir/.proxy-version")"
+if [ -z "$old_proxy_version" ]; then
+    echo "missing .proxy-version" >&2
+    exit 1
+fi
+
+# Checkout the linkerd2-proxy repo to resolve the new proxy version to a SHA
+# and obtain the commit log since the prior version.
+tmp=$(mktemp -d -t l2-proxy.XXX)
+git clone --depth=100 https://github.com/linkerd/linkerd2-proxy "$tmp"
+cd "$tmp"
+
+sha="$(git rev-parse "${new_proxy_revision}")"
+new_proxy_version="${sha:0:7}"
+
+if ! git-rev-parse --verify --quiet "${old_proxy_version}" ; then
+    echo "The old proxy-version ${old_proxy_version} does not exist in the last 100 proxy commits." >&2
+    exit 2
+fi
+
+echo "proxy: Update proxy to ${new_proxy_revision}" >"$tmp/commit.template"
+echo "" >>"$tmp/commit.template"
+
+git log --pretty=oneline --abbrev-commit --reverse "${old_proxy_version}..${new_proxy_version}" | \
+sed -E -e 's,^,* linkerd/linkerd2-proxy#,' \
+    -e 's, \((#[0-9]+)\), (linkerd/linkerd2-proxy\1),' \
+    >>"$tmp/commit.template"
+
+cd "$rootdir"
+echo "$new_proxy_version" >"$rootdir/.proxy-version"
+
+git commit --file="$tmp/commit.template" --edit -- "$rootdir/.proxy-version"
+
+rm -rf "$tmp"
+
+git --no-pager show HEAD