Implement HOMEBREW_FORCE_BREWED_GIT

Because of this messing with the user's path:
https://github.com/Homebrew/brew/blob/
    efc02899c851c62c9ce0d15dea9a231575d7d774/bin/brew#L68
brew uses /usr/bin/git over brewed git, even when the former is
problematically old.
There may also be other reasons a user prefers to use brewed git.

There was already a HOMEBREW_FORCE_BREWED_CURL option and a
HOMEBREW_SYSTEM_CURL_TOO_OLD check to set it. This mostly copies those
to implement HOMEBREW_FORCE_BREWED_GIT & HOMEBREW_SYSTEM_GIT_TOO_OLD.

See also: https://github.com/Linuxbrew/brew/issues/736

Library/Homebrew/brew.sh

@@ -102,6 +102,7 @@ then
   if [[ "$HOMEBREW_MACOS_VERSION_NUMERIC" -lt "100900" ]]
   then
     HOMEBREW_SYSTEM_GIT_TOO_OLD="1"
+    HOMEBREW_FORCE_BREWED_GIT="1"
   fi
 
   if [[ -z "$HOMEBREW_CACHE" ]]
@@ -135,6 +136,15 @@ else
   HOMEBREW_CURL="curl"
 fi
 
+if [[ -n "$HOMEBREW_FORCE_BREWED_GIT" &&
+      -x "$HOMEBREW_PREFIX/opt/git/bin/git" ]] &&
+         "$HOMEBREW_PREFIX/opt/git/bin/git" --version >/dev/null
+then
+  HOMEBREW_GIT="$HOMEBREW_PREFIX/opt/git/bin/git"
+else
+  HOMEBREW_GIT="git"
+fi
+
 HOMEBREW_USER_AGENT="$HOMEBREW_PRODUCT/$HOMEBREW_USER_AGENT_VERSION ($HOMEBREW_SYSTEM; $HOMEBREW_PROCESSOR $HOMEBREW_OS_USER_AGENT_VERSION)"
 HOMEBREW_CURL_VERSION="$("$HOMEBREW_CURL" --version 2>/dev/null | head -n1 | awk '{print $1"/"$2}')"
 HOMEBREW_USER_AGENT_CURL="$HOMEBREW_USER_AGENT $HOMEBREW_CURL_VERSION"

Library/Homebrew/cmd/update.sh

@@ -384,7 +384,8 @@ EOS
   fi
 
   if ! git --version &>/dev/null ||
-       [[ -n "$HOMEBREW_SYSTEM_GIT_TOO_OLD" &&
+     [[ ( -n "$HOMEBREW_SYSTEM_GIT_TOO_OLD" ||
+          -n "$HOMEBREW_FORCE_BREWED_GIT" ) &&
         ! -x "$HOMEBREW_PREFIX/opt/git/bin/git" ]]
   then
     # we cannot install brewed git if homebrew/core is unavailable.

Library/Homebrew/manpages/brew.1.md.erb

@@ -184,6 +184,10 @@ Note that environment variables must have a value set to be detected. For exampl
 
     Set this to force Homebrew to use a particular git binary.
 
+  * `HOMEBREW_FORCE_BREWED_GIT`:
+    If set, Homebrew will use a Homebrew-installed `git` rather than the
+    system version.
+
   * `HOMEBREW_GITHUB_API_TOKEN`:
     A personal access token for the GitHub API, which you can create at
     <https://github.com/settings/tokens>. If set, GitHub will allow you a

docs/Manpage.md

@@ -1167,6 +1167,10 @@ Note that environment variables must have a value set to be detected. For exampl
 
     Set this to force Homebrew to use a particular git binary.
 
+  * `HOMEBREW_FORCE_BREWED_GIT`:
+    If set, Homebrew will use a Homebrew-installed `git` rather than the
+    system version.
+
   * `HOMEBREW_GITHUB_API_TOKEN`:
     A personal access token for the GitHub API, which you can create at
     <https://github.com/settings/tokens>. If set, GitHub will allow you a