Reduce number of git calls when displaying prompt (#3795)

The avit theme's _git_time_since_commit function was running git twice.
Reduce it with a single call to `git log`, checking the exit code for
success.

themes/avit.zsh-theme

@@ -50,9 +50,7 @@ function _ruby_version() {
 # use a neutral color, otherwise colors will vary according to time.
 function _git_time_since_commit() {
 # Only proceed if there is actually a commit.
-  if git log -1 > /dev/null 2>&1; then
-    # Get the last commit.
-    last_commit=$(git log --pretty=format:'%at' -1 2> /dev/null)
+  if last_commit=$(git log --pretty=format:'%at' -1 2> /dev/null); then
     now=$(date +%s)
     seconds_since_last_commit=$((now-last_commit))