Improve the description of GIT_PS1_SHOWUPSTREAM
Improve the description of GIT_PS1_SHOWUPSTREAM

Describe what '=' means in the output of __git_ps1 when using
GIT_PS1_SHOWUPSTREAM, which was not previously described.

Signed-off-by: Jonathan "Duke" Leto <3692bfa45759a67d83aedf0045f6cb635a966abf@leto.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git