Makefile: be silent when only GIT_USER_AGENT changes
Makefile: be silent when only GIT_USER_AGENT changes

To avoid noise during builds, unlike the GIT-CFLAGS rule which prints
"* new build flags or prefix" so the operator knows why all files are
being rebuilt when it changes, GIT-USER-AGENT generation is silent.

If this code breaks and a target depending on GIT-USER-AGENT ends up
being rebuilt when it shouldn't be, the full dependency chain can be
retrieved with "make --debug=b".

Signed-off-by: Jonathan Nieder <b57189c5f2fd5b3daf3350a77fde90def1080fa5@gmail.com>
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