Update current Xcode target's build number with the number of commits on a specified branch. http://tgoode.com/2014/06/05/sensible-way-increment-bundle-version-cfbundleversion-xcode/

update_build_number.sh

#!/bin/bash

#  update_build_number.sh
#  Usage: `update_build_number.sh [branch]`
#  Run this script after the 'Copy Bundle Resources' build phase
#  Ref: http://tgoode.com/2014/06/05/sensible-way-increment-bundle-version-cfbundleversion-xcode/

branch=${1:-'master'}
buildNumber=$(expr $(git rev-list $branch --count) - $(git rev-list HEAD..$branch --count))
echo "Updating build number to $buildNumber using branch '$branch'."
/usr/libexec/PlistBuddy -c "Set :CFBundleVersion $buildNumber" "${TARGET_BUILD_DIR}/${INFOPLIST_PATH}"
if [ -f "${BUILT_PRODUCTS_DIR}/${WRAPPER_NAME}.dSYM/Contents/Info.plist" ]; then
  /usr/libexec/PlistBuddy -c "Set :CFBundleVersion $buildNumber" "${BUILT_PRODUCTS_DIR}/${WRAPPER_NAME}.dSYM/Contents/Info.plist"
fi

To use: add this as a 'Run Script' after the 'Copy Bundle Resources' build script in Xcode. If no argument is provided (to specify which branch to compare against), master is used by default.

Update: now updating the dSYM Info.plist as well. Thanks to this comment by @dismory.

Update: make sure the dSYM Info.plist file exists before trying to update it. Thanks to this comment by Motti Shneor.

If you don't want to anchor the build number to a branch, you can use this to calculate it:
buildNumber=$(git rev-list HEAD --count)

@airdrummingfool, with Xcode 10, ${BUILT_PRODUCTS_DIR}/${WRAPPER_NAME}.dSYM seems to be always missing. Good thing there is an if -f in the script, but I wonder if we need to change something for Xcode 10?