download_and_compile.sh: add option --override-repo

This option allows one to override the repository from which a given
component is to be cloned (it doesn't do anything when the repository
for a component is simply updated).

Example that downloads OSG 3.4.2 from James' repository (it has
FlightGear-specific patches applied):

  download_and_compile.sh -j$(nproc) -s --cleanup \
    --override-repo OSG=GitHub:github.com/zakalawe/osg.git \
    --component-branch OSG=fgfs-342-1 SIMGEAR FGFS DATA OSG

download_and_compile.sh

@@ -873,7 +873,7 @@ Available options:
                 default protocol to use for 'git clone' (https, git or ssh)
       --git-clone-site-params=SITE=PROTOCOL[:USERNAME]
                 use PROTOCOL as USERNAME when cloning a Git repository located
-                at SITE (sample sites: 'sourceforge', 'github'; valid
+                at SITE (sample sites: 'SourceForge', 'GitHub'; valid
                 protocols: 'ssh', 'https', 'git'; USERNAME is required when
                 using 'ssh'). You may pass this option several times with
                 different sites.
@@ -901,6 +901,17 @@ Available options:
                 corresponding versions for other components)
   -s            compile the latest stable release of FlightGear (and
                 corresponding versions of the other components)
+      --override-repo=COMPONENT=SITE:ADDRESS
+                Override the repository from which COMPONENT is fetched when
+                its repository is cloned (initial retrieval). SITE is defined
+                as in --git-clone-site-params (e.g., 'SourceForge' or 'GitHub').
+                ADDRESS should not start with a protocol specification. This
+                option may be given several times.
+
+                Example: in order to download OSG from the fgfs-342-1 branch of
+                the repository at https://github.com/zakalawe/osg.git, you can
+                use --override-repo OSG=GitHub:github.com/zakalawe/osg.git
+                and --component-branch OSG=fgfs-342-1
       --component-branch=COMPONENT=BRANCH
                 Override the default branch for COMPONENT. For the specified
                 component, this overrides the effect of options -s, --lts and
@@ -1048,7 +1059,7 @@ TEMP=$($getopt -o '+shc:p:a:d:r:j:O:ib:' \
   --longoptions cleanup,git-clone-default-proto:,git-clone-site-params: \
   --longoptions package-manager:,sudo:,ignore-intercomponent-deps,compositor \
   --longoptions component-branch:,sg-cmake-arg:,fg-cmake-arg:,non-interactive \
-  --longoptions lts,old-lts,help,version \
+  --longoptions lts,old-lts,override-repo:,help,version \
   -n "$PROGNAME" -- "$@")
 
 case $? in
@@ -1150,6 +1161,32 @@ while true; do
       fi
       shift 2
       ;;
+    --override-repo)
+      if [[ $2 =~ ^([[:alnum:]]+)=([[:alnum:]]+):(.+) ]]; then
+        component=${BASH_REMATCH[1]^^}      # convert to uppercase
+        site=${BASH_REMATCH[2],,}           # convert to lowercase
+        address=${BASH_REMATCH[3]}
+
+        if [[ $address =~ ^(https?|(([[:alnum:]]+)@)?ssh):/ ]]; then
+          echo "When using --override-repo COMPONENT=SITE:ADDRESS, the" \
+               "ADDRESS part should not" >&2
+          echo "start with a protocol specification like 'https://'; it" \
+               "should start with the" >&2
+          echo "hostname." >&2
+          exit 1
+        fi
+
+        REPO_ADDRESS["$component"]=$address
+        REPO_SITE["$component"]=$site
+        unset -v component site address
+      else
+        echo "Invalid value passed to option --override-repo: '$2'." >&2
+        echo "The correct syntax is --override-repo COMPONENT=SITE:ADDRESS" >&2
+        echo "(or equivalently, --override-repo=COMPONENT=SITE:ADDRESS)." >&2
+        exit 1
+      fi
+      shift 2
+      ;;
     --package-manager) PKG_MGR="$2"; shift 2 ;;
     --sudo) SUDO="$2"; shift 2 ;;
     --ignore-intercomponent-deps) IGNORE_INTERCOMPONENT_DEPS="y"; shift ;;