completion/git: expand search range

- Remove limitation on OS.
- Add search for Mac OS X developer tools locations by using `xcrun` instead of trying to guess paths.
- Add search for locations based on path of `$GIT_EXE` (set by `lib/theme`).

completion/available/git.completion.bash

@@ -1,28 +1,31 @@
-#!/usr/bin/env bash
-
-# Only operate on MacOS since there are no linux paths
-if [[ "$OSTYPE" != 'darwin'* ]] ; then
-  _log_warning "unsupported operating system - only 'Darwin' is supported"
-  return 0
-fi
+# shellcheck shell=bash
+#
+# Locate and load completions for `git`.
 
 # Make sure git is installed
-_command_exists git || return 0
+_command_exists git || return
 
 # Don't handle completion if it's already managed
-if complete -p git &>/dev/null ; then
+if complete -p git &>/dev/null; then
   _log_warning "completion already loaded - this usually means it is safe to stop using this completion"
   return 0
 fi
 
-_git_bash_completion_found=false
+_git_bash_completion_xcrun_git=
+if _command_exists xcrun; then
+	_git_bash_completion_xcrun_git="$(xcrun --find git)"
+fi
 _git_bash_completion_paths=(
+  # Standard locations
+  "${GIT_EXE%/*}/../share/git-core/git-completion.bash"
+  "${GIT_EXE%/*}/../share/git-core/contrib/completion/git-completion.bash"
+  "${GIT_EXE%/*}/../etc/bash_completion.d/git-completion.bash"
   # MacOS non-system locations
-  '/Library/Developer/CommandLineTools/usr/share/git-core/git-completion.bash'
-  '/Applications/Xcode.app/Contents/Developer/usr/share/git-core/git-completion.bash'
+  "${_git_bash_completion_xcrun_git%/bin/git}/share/git-core/git-completion.bash"
 )
 
 # Load the first completion file found
+_git_bash_completion_found=false
 for _comp_path in "${_git_bash_completion_paths[@]}" ; do
   if [[ -r "$_comp_path" ]] ; then
     _git_bash_completion_found=true
@@ -35,5 +38,4 @@ done
 if [[ "${_git_bash_completion_found}" == false ]]; then
   _log_warning "no completion files found - please try enabling the 'system' completion instead."
 fi
-unset _git_bash_completion_paths
-unset _git_bash_completion_found
+unset "${!_git_bash_completion@}"