brew.sh: improve user agent with no git repo.

`Homebrew/>1.0.0 (no git repository) (Macintosh; Intel macOS 10.7.5)...)
reads pretty weirdly in a user agent and I've had complaints that `>`
may be an invalid character in some cases.

Library/Homebrew/brew.sh

 HOMEBREW_VERSION="$(git -C "$HOMEBREW_REPOSITORY" describe --tags --dirty 2>/dev/null)"
+HOMEBREW_USER_AGENT_VERSION="$HOMEBREW_VERSION"
 if [[ -z "$HOMEBREW_VERSION" ]]
 then
   HOMEBREW_VERSION=">1.0.0 (no git repository)"
+  HOMEBREW_USER_AGENT_VERSION="1.X.Y"
 fi
 
 # A depth of 1 means this command was directly invoked by a user.
@@ -109,7 +111,7 @@ else
   [[ -n "$HOMEBREW_LINUX" ]] && HOMEBREW_OS_VERSION="$(lsb_release -sd 2>/dev/null)"
   : "${HOMEBREW_OS_VERSION:=$(uname -r)}"
 fi
-HOMEBREW_USER_AGENT="$HOMEBREW_PRODUCT/$HOMEBREW_VERSION ($HOMEBREW_SYSTEM; $HOMEBREW_PROCESSOR $HOMEBREW_OS_VERSION)"
+HOMEBREW_USER_AGENT="$HOMEBREW_PRODUCT/$HOMEBREW_USER_AGENT_VERSION ($HOMEBREW_SYSTEM; $HOMEBREW_PROCESSOR $HOMEBREW_OS_VERSION)"
 HOMEBREW_CURL_VERSION="$("$HOMEBREW_CURL" --version 2>/dev/null | head -n1 | /usr/bin/awk '{print $1"/"$2}')"
 HOMEBREW_USER_AGENT_CURL="$HOMEBREW_USER_AGENT $HOMEBREW_CURL_VERSION"