Cirrus: Git cache mutates too much

[JeremyRand]

When uploading the Git cache, I see this a lot:

[11:05:10.404] SHA for cache folders (/tmp/cirrus-ci-build/git_clones) is 'f7a7f6a796f55a086657d4067c0a71da80e4ab34a4e91b366aed00027406769d'
[11:05:10.404] Cache git_release_osx_x86_64 has changed!
[11:05:10.404] List of changes for cache folders (/tmp/cirrus-ci-build/git_clones):
[11:05:10.457] modified: ncp11/.git/index
[11:05:10.457] modified: goeasyconfig/.git/index
[11:05:10.457] modified: gowebsocket/.git/index
[11:05:10.457] modified: cmake/.git/index
[11:05:10.457] modified: gobtcd/.git/index
[11:05:10.457] modified: cctools/.git/index
[11:05:10.457] modified: godns/.git/index
[11:05:10.457] modified: goservice/.git/index
[11:05:10.458] modified: ncprop279/.git/index
[11:05:10.458] modified: goisatty/.git/index
[11:05:10.458] modified: gobtcd2/.git/index
[11:05:10.458] modified: goncrpcclient/.git/index
[11:05:10.458] modified: goansicolor/.git/index
[11:05:10.458] modified: gotext/.git/index
[11:05:10.458] modified: gogroupcache/.git/index
[11:05:10.458] modified: gosocks/.git/index
[11:05:10.458] modified: gounits/.git/index
[11:05:10.458] modified: ninja/.git/index
[11:05:10.458] modified: gobtcutil/.git/index
[11:05:10.458] modified: gosvcutils/.git/index
[11:05:10.458] modified: goconfigurable/.git/index
[11:05:10.458] modified: goxcryptoripemd160/.git/index
[11:05:10.458] modified: gokingpin/.git/index
[11:05:10.458] modified: goxcryptoed25519/.git/index
[11:05:10.458] modified: gopkcs11/.git/index
[11:05:10.458] modified: gosplicesign/.git/index
[11:05:10.458] modified: gopflag/.git/index
[11:05:10.458] modified: gotemplate/.git/index
[11:05:10.458] modified: goxnet/.git/index
[11:05:10.458] modified: gobtclog/.git/index
[11:05:10.458] modified: gotlsrestrictnss/.git/index
[11:05:10.458] modified: libtapi/.git/index
[11:05:10.458] modified: goncbtcjson/.git/index
[11:05:10.458] modified: ncdns/.git/index
[11:05:10.458] modified: gocrosssignnameconstraint/.git/index
[11:05:10.458] modified: goxnetip/.git/index
[11:05:10.458] modified: gomadns/.git/index
[11:05:10.458] modified: godegoutils/.git/index
[11:05:10.458] modified: goxlog/.git/index
[11:05:10.458] modified: goxsys/.git/index
[11:05:10.458] modified: gosystemd/.git/index
[11:05:10.458] modified: gobuildinfo/.git/index
[11:05:10.458] modified: gopkcs11mod/.git/index
[11:05:10.458] modified: godexlogconfig/.git/index
[11:05:10.458] modified: goxsysunix/.git/index
[11:05:10.458] modified: gopretty/.git/index
[11:05:10.458] modified: gotoml/.git/index
[11:05:15.398] git_release_osx_x86_64 cache size is 418Mb.
[11:05:15.398] Uploading cache git_release_osx_x86_64...

The resulting upload took 17 seconds, all because Git mutates its index file in all repos that we touched. Is there a way around this? Will Git get unhappy if we, say, just delete all the index files prior to uploading the cache?

[JeremyRand]

Deleting index seems to cause Bad Things (i.e. it stages all files for removal). Not sure if there's another way to prevent it from mutating.