Added support to specify '-b branch@commit'

lib/git-subrepo

@@ -118,6 +118,7 @@ main() {
 
   local subrepo_remote=         # Remote url for subrepo's upstream repo
   local subrepo_branch=         # Upstream branch to clone/push/pull
+  local subrepo_branch_commit=  # Upstream branch commit to clone
   local subrepo_commit=         # Upstream HEAD from previous clone/pull
   local subrepo_parent=         # Local commit from before previous clone/pull
   local subrepo_former=         # A retired gitrepo key that might still exist
@@ -205,7 +206,11 @@ command:clone() {
   local re=
   $force_wanted && re=re
   local remote=$subrepo_remote
-  say "Subrepo '$remote' ($subrepo_branch) ${re}cloned into '$subdir'."
+  local bc=$subrepo_branch
+  if [ -n "$subrepo_branch_commit" ]; then
+    bc=$subrepo_branch@$subrepo_branch_commit
+  fi
+  say "Subrepo '$remote' ($bc) ${re}cloned into '$subdir'."
 }
 
 # `git subrepo init <subdir>` command:
@@ -236,7 +241,11 @@ command:pull() {
 
   subrepo:pull
   if OK; then
-    say "Subrepo '$subdir' pulled from '$subrepo_remote' ($subrepo_branch)."
+    local bc=$subrepo_branch
+    if [ -n "$subrepo_branch_commit" ]; then
+      bc=$subrepo_branch@$subrepo_branch_commit
+    fi
+    say "Subrepo '$subdir' pulled from '$subrepo_remote' ($bc)."
   elif [[ $CODE -eq -1 ]]; then
     say "Subrepo '$subdir' is up to date."
   elif [[ $CODE -eq 1 ]]; then
@@ -274,7 +283,11 @@ command:fetch() {
     say "Ignored '$subdir', no remote."
   else
     subrepo:fetch
-    say "Fetched '$subdir' from '$subrepo_remote' ($subrepo_branch)."
+    local bc=$subrepo_branch
+    if [ -n "$subrepo_branch_commit" ]; then
+      bc=$subrepo_branch@$subrepo_branch_commit
+    fi
+    say "Fetched '$subdir' from '$subrepo_remote' ($bc)."
   fi
 }
 
@@ -727,6 +740,13 @@ subrepo:fetch() {
   RUN git fetch --no-tags --quiet "$subrepo_remote" "$subrepo_branch"
   OK || return
 
+  if [[ -n "$subrepo_branch_commit" ]]; then
+    reset-upstream-subrepo-branch-commit
+    o "Re-fetch the upstream: $subrepo_remote ($subrepo_branch@$subrepo_branch_commit)."
+    RUN git fetch --no-tags --quiet "$subrepo_remote" "$subrepo_branch_commit"
+    OK || return
+  fi
+
   o "Get the upstream subrepo HEAD commit."
   OUT=true RUN git rev-parse FETCH_HEAD^0
   upstream_head_commit=$output
@@ -1093,8 +1113,11 @@ get-command-options() {
       -a) all_wanted=true ;;
       -A) ALL_wanted=true
           all_wanted=true ;;
-      -b) subrepo_branch=$1
-          override_branch=$1
+      -b) subrepo_branch=${1%%@*}
+          override_branch=${1%%@*}
+          if [ "${subrepo_branch}" != "${1}" ]; then
+            subrepo_branch_commit=${1#*@}
+          fi
           commit_msg_args+=("--branch=$1")
           shift ;;
       -e) edit_wanted=true ;;
@@ -1650,6 +1673,25 @@ add-subrepo() {
   subrepos+=("$1")
 }
 
+# Determine and set real commit behind branch@commit request
+reset-upstream-subrepo-branch-commit() {
+  local branch_commit
+  OUT=true RUN git rev-parse FETCH_HEAD^0
+  branch_commit=$output
+  [[ -n "$branch_commit" ]] ||
+    error "Problem finding head commit in upstream branch '${subrepo_branch}'."
+
+  if [[ ! "${branch_commit}" =~ ^$subrepo_branch_commit ]]; then
+    OUT=true FAIL=false RUN git rev-list --parents "$subrepo_branch_commit".."$branch_commit"
+    branch_commit=$(
+      echo "$output" | sed -e '$!d' -e 's/.* //'
+    )
+    [[ -n "$branch_commit" ]] ||
+      error "Problem finding '${subrepo_branch_commit}' commit in upstream branch '${subrepo_branch}'."
+  fi
+  subrepo_branch_commit=$branch_commit
+}
+
 # Determine the upstream's default head branch:
 get-upstream-head-branch() {
   local remotes branch