git absorb seems to ignore git CLI config override options

[gibson042]

The git CLI supports overriding configuration with options like -c absorb.fixupTargetAlwaysSHA, but these appear to be ignored by git-absorb (unlike GIT_CONFIG_GLOBAL/GIT_CONFIG_SYSTEM environment variables, which seem to be respected):

$ AWK_SCAN_OUTPUT='  
/would have committed/ {
  if(match($0, /[0-9a-fA-F]{8,}/)) by_hash++; else by_subject++;
}
END {
  printf "%d by hash, %d by subject\n", by_hash, by_subject;
}
'

$ git absorb --dry-run 2>&1 | awk "$AWK_SCAN_OUTPUT"
0 by hash, 2 by subject

$ git -c absorb.fixupTargetAlwaysSHA absorb --dry-run 2>&1 | awk "$AWK_SCAN_OUTPUT"
0 by hash, 2 by subject

$ printf '[absorb]\n    fixupTargetAlwaysSHA = true\n' > /tmp/dummy-gitconfig

$ cat /tmp/dummy-gitconfig 
[absorb]
    fixupTargetAlwaysSHA = true

$ GIT_CONFIG_GLOBAL=/tmp/dummy-gitconfig git absorb --dry-run 2>&1 | awk "$AWK_SCAN_OUTPUT"
2 by hash, 0 by subject

Reading the source code suggests to me that the issue is probably in either git2 or in this code's use of it.

[blairconrad]

Possible upstream issue? libgit2/libgit2#3854