Allow git to merge Cargo.lock

This commit backs out rust-lang#46539 in order to fully leverage rust-lang#63579 where
`git` should be able to merge `Cargo.lock` nowadays with only minimal
conflicts.

Author: alexcrichton

.gitattributes

@@ -8,7 +8,7 @@
 src/etc/installer/gfx/* binary
 *.woff binary
 src/vendor/** -text
-Cargo.lock -merge linguist-generated=false
+Cargo.lock linguist-generated=false
 
 # Older git versions try to fix line endings on images, this prevents it.
 *.png binary