completion: loosen and document the requirement around completing alias

Recently we started to tell users to spell ": git foo ;" with
space(s) around 'foo' for an alias to be completed similarly
to the 'git foo' command.  It however is easy to also allow users to
spell it in a more natural way with the semicolon attached to 'foo',
i.e. ": git foo;".  Also, add a comment to note that 'git' is optional
and writing ": foo;" would complete the alias just fine.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 47fd664..477ef81 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -28,7 +28,8 @@
 # completion style.  For example '!f() { : git commit ; ... }; f' will
 # tell the completion to use commit completion.  This also works with aliases
 # of form "!sh -c '...'".  For example, "!sh -c ': git commit ; ... '".
-# Be sure to add a space between the command name and the ';'.
+# Note that "git" is optional --- '!f() { : commit; ...}; f' would complete
+# just like the 'git commit' command.
 #
 # If you have a command that is not part of git, but you would still
 # like completion, you can use __git_complete:
@@ -1183,7 +1184,7 @@
 			:)	: skip null command ;;
 			\'*)	: skip opening quote after sh -c ;;
 			*)
-				cur="$word"
+				cur="${word%;}"
 				break
 			esac
 		done

t/t9902-completion.sh

diff --git a/t/t9902-completion.sh b/t/t9902-completion.sh
index 47e20fb..a7c3b4e 100755
--- a/t/t9902-completion.sh
+++ b/t/t9902-completion.sh
@@ -2464,6 +2464,24 @@
 	EOF
 '
 
+test_expect_success 'completion used <cmd> completion for alias: !f() { : <cmd> ; ... }' '
+	test_config alias.co "!f() { : checkout ; if ... } f" &&
+	test_completion "git co m" <<-\EOF
+	main Z
+	mybranch Z
+	mytag Z
+	EOF
+'
+
+test_expect_success 'completion used <cmd> completion for alias: !f() { : <cmd>; ... }' '
+	test_config alias.co "!f() { : checkout; if ... } f" &&
+	test_completion "git co m" <<-\EOF
+	main Z
+	mybranch Z
+	mytag Z
+	EOF
+'
+
 test_expect_success 'completion without explicit _git_xxx function' '
 	test_completion "git version --" <<-\EOF
 	--build-options Z