bootstrap.bash: only fetch git revision if we need it
bootstrap.bash: only fetch git revision if we need it

Updates #22912
Fixes #23610

Change-Id: Id1c91fc3f040412d5931dba40e430685793f8eea
Reviewed-on: https://go-review.googlesource.com/90715
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,CAFxX/go,codestation/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,golang/go,Cofyc/go,akutz/go,akutz/go,akutz/go,Cofyc/go,golang/go,akutz/go,Cofyc/go,josharian/go,Cofyc/go,golang/go,Cofyc/go,CAFxX/go,codestation/go,christopher-henderson/Go,nawawi/go,codestation/go,nawawi/go,golang/go,CAFxX/go,akutz/go,josharian/go,golang/go,codestation/go,josharian/go,josharian/go,nawawi/go,cloudflare/go,cloudflare/go,cloudflare/go,josharian/go,golang/go,CAFxX/go,akutz/go,codestation/go,Cofyc/go,Cofyc/go,golang/go,nawawi/go,cloudflare/go,Cofyc/go,cloudflare/go,josharian/go,akutz/go,nawawi/go,codestation/go,akutz/go,josharian/go,codestation/go,josharian/go,cloudflare/go,codestation/go