Handle undefined __git_complete (#1055)

As identified in #1011 (comment), there are cases where `__git_complete` is undefined because it is loaded dynamically by bash_completions.

As suggested in #1011 (comment), using `pkg-config` to find `git-completion` script, which defines `__git_complete`.  And using approach similar to https://github.com/felipec/git-completion/blob/master/git-completion.zsh#L28-L49.

Fixes #1011

contrib/tig-completion.bash

@@ -1,5 +1,5 @@
 # bash/zsh completion for tig
-# 
+#
 # Copyright (C) 2019 Roland Hieber, Pengutronix
 # Copyright (C) 2007-2010 Jonas fonseca
 #
@@ -29,6 +29,27 @@
 #       at source time then all lookups will be done on demand,
 #       which may be slightly slower.
 
+#tig-completion requires __git_complete
+#* If not defined, source git completions script so __git_complete is available
+if ! declare -f __git_complete &>/dev/null; then
+	_bash_completion=$(pkg-config --variable=completionsdir bash-completion 2>/dev/null) ||
+		_bash_completion='/usr/share/bash-completion/completions/'
+			_locations=(
+				"$(dirname "${BASH_SOURCE[0]%:*}")"/git-completion.bash #in same dir as this
+				"$HOME/.local/share/bash-completion/completions/git"
+				"$_bash_completion/git"
+				'/etc/bash_completion.d/git' # old debian
+			)
+			for _e in "${_locations[@]}"; do
+				# shellcheck disable=1090
+				test -f "$_e" && . "$_e" && break
+			done
+			unset _bash_completion _locations _e
+			if ! declare -f __git_complete &>/dev/null; then
+				return #silently return without completions
+			fi
+fi
+
 __tig_options="
 	-v --version
 	-h --help